diff --git a/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index.rb b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index.rb
index c70d77bb2..229a41f20 100644
--- a/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index.rb
+++ b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index.rb
@@ -7,6 +7,7 @@
 # frozen_string_literal: true
 
 require "elastic_graph/admin/cluster_configurator/action_reporter"
+require "elastic_graph/admin/index_definition_configurator/mapping_update"
 require "elastic_graph/datastore_core/index_config_normalizer"
 require "elastic_graph/indexer/hash_differ"
 require "elastic_graph/support/hash_util"
@@ -32,9 +33,8 @@ def initialize(datastore_client, index, env_agnostic_index_config, output)
         # and the state of the index in the datastore, does one of the following:
         #
         #   - If the index did not already exist: creates the index with the desired mappings and settings.
-        #   - If the desired mapping has fewer fields than what is in the index: raises an exception,
-        #     because the datastore provides no way to remove fields from a mapping and it would be confusing
-        #     for this method to silently ignore the issue.
+        #   - If the desired mapping has fewer fields than what is in the index: leaves the existing fields
+        #     alone, because the datastore provides no way to remove fields from a mapping.
         #   - If the settings have desired changes: updates the settings, restoring any setting that
         #     no longer has a desired value to its default.
         #   - If the mapping has desired changes: updates the mappings.
@@ -75,14 +75,9 @@ def create_new_index
         end
 
         def update_mapping
-          @datastore_client.put_index_mapping(index: @index.name, body: desired_mapping)
+          @datastore_client.put_index_mapping(index: @index.name, body: desired_mapping_for_update)
           action_description = "Updated mappings for index `#{@index.name}`:\n#{mapping_diff}"
 
-          if mapping_removals.any?
-            action_description += "\n\nNote: the extra fields listed here will not actually get removed. " \
-              "Mapping removals are unsupported (but ElasticGraph will leave them alone and they'll cause no problems)."
-          end
-
           report_action action_description
         end
 
@@ -100,10 +95,6 @@ def index_exists?
           !current_config.empty?
         end
 
-        def mapping_removals
-          @mapping_removals ||= mapping_fields_from(current_mapping) - mapping_fields_from(desired_mapping)
-        end
-
         def mapping_type_changes
           @mapping_type_changes ||= begin
             flattened_current = Support::HashUtil.flatten_and_stringify_keys(current_mapping)
@@ -116,7 +107,7 @@ def mapping_type_changes
         end
 
         def has_mapping_updates?
-          current_mapping != desired_mapping
+          current_mapping != desired_mapping_for_update
         end
 
         def settings_updates
@@ -127,15 +118,8 @@ def settings_updates
           end
         end
 
-        def mapping_fields_from(mapping_hash, prefix = "")
-          (mapping_hash["properties"] || []).flat_map do |key, params|
-            field = prefix + key
-            if params.key?("properties")
-              [field] + mapping_fields_from(params, "#{field}.")
-            else
-              [field]
-            end
-          end
+        def desired_mapping_for_update
+          @desired_mapping_for_update ||= MappingUpdate.build_mapping_update(desired: desired_mapping, current: current_mapping)
         end
 
         def desired_mapping
@@ -178,7 +162,7 @@ def current_config
         end
 
         def mapping_diff
-          @mapping_diff ||= Indexer::HashDiffer.diff(current_mapping, desired_mapping) || "(no diff)"
+          @mapping_diff ||= Indexer::HashDiffer.diff(current_mapping, desired_mapping_for_update) || "(no diff)"
         end
 
         def settings_diff
diff --git a/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index_template.rb b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index_template.rb
index bce4ea0bf..40b0bc0a4 100644
--- a/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index_template.rb
+++ b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/for_index_template.rb
@@ -8,6 +8,7 @@
 
 require "elastic_graph/admin/cluster_configurator/action_reporter"
 require "elastic_graph/admin/index_definition_configurator/for_index"
+require "elastic_graph/admin/index_definition_configurator/mapping_update"
 require "elastic_graph/datastore_core/index_config_normalizer"
 require "elastic_graph/indexer/hash_differ"
 require "elastic_graph/support/hash_util"
@@ -36,9 +37,9 @@ def initialize(datastore_client, index_template, env_agnostic_index_config_paren
         # and the state of the index in the datastore, does one of the following:
         #
         #   - If the index did not already exist: creates the index with the desired mappings and settings.
-        #   - If the desired mapping has fewer fields than what is in the index: raises an exception,
-        #     because the datastore provides no way to remove fields from a mapping and it would be confusing
-        #     for this method to silently ignore the issue.
+        #   - If the desired mapping has fewer fields than what is in the index template: preserves fields
+        #     that the schema definition still references (via `deleted_field`/`renamed_from` declarations)
+        #     and drops fields whose last schema reference has been removed (see `put_index_template`).
         #   - If the settings have desired changes: updates the settings, restoring any setting that
         #     no longer has a desired value to its default.
         #   - If the mapping has desired changes: updates the mappings.
@@ -66,23 +67,48 @@ def validate
 
         private
 
+        # Creates or updates the index template. Fields that exist on the current template but not in the
+        # desired configuration are dropped only when it is provably safe. New rollover indices are
+        # auto-created from the template at indexing time with `dynamic: strict` mappings, so dropping a
+        # field that something can still write would cause indexing failures on those new indices--we have
+        # previously had a near-SEV from dropping a template field while deployed indexers were still
+        # running an old version of the code that used it. So, as long as a field is referenced by a
+        # `deleted_field` or `renamed_from` declaration in the schema definition (meaning an old JSON
+        # schema version may still reference it, and an indexer deployed with an older schema version or a
+        # replayed old event may still write it), we preserve it via `field_paths_protected_from_removal`.
+        # Once the last reference is removed from the schema definition--which `eg-schema_def` only
+        # sanctions once no JSON schema version references the field--the field is dropped on the next
+        # admin run, keeping templates from accumulating stale fields forever.
         def put_index_template
-          desired_template_config_payload = Support::HashUtil.deep_merge(
-            desired_config_parent,
-            {"template" => {"mappings" => merge_properties(desired_mapping, current_mapping)}}
-          )
-
-          action_description = "Updated index template: `#{@index_template.name}`:\n#{config_diff}"
-
-          if mapping_removals.any?
-            action_description += "\n\nNote: the extra fields listed here will not actually get removed. " \
-              "Mapping removals are unsupported (but ElasticGraph will leave them alone and they'll cause no problems)."
+          action_description = if index_template_exists?
+            "Updated index template: `#{@index_template.name}`:\n#{config_diff}#{removed_fields_note}"
+          else
+            "Created index template: `#{@index_template.name}`"
           end
 
-          @datastore_client.put_index_template(name: @index_template.name, body: desired_template_config_payload)
+          @datastore_client.put_index_template(name: @index_template.name, body: desired_config_parent_for_update)
           report_action action_description
         end
 
+        # Dropped fields show up as deletions in the reported diff, but we also call them out explicitly
+        # since a field removal is the part of a mapping update most worth a second look from operators.
+        def removed_fields_note
+          removed_fields = mapping_field_paths_of(current_mapping) - mapping_field_paths_of(desired_mapping_for_update)
+          return "" if removed_fields.empty?
+
+          "\n\n" + <<~EOS.chomp
+            Removed #{removed_fields.size} stale field(s) from index template `#{@index_template.name}` that are no longer referenced by the schema: #{removed_fields.join(", ")}.
+            New rollover indices created from this template will reject documents that still contain these fields (mappings are `dynamic: strict`).
+          EOS
+        end
+
+        def mapping_field_paths_of(mapping, parent_path: "")
+          mapping.fetch("properties", MappingUpdate::EMPTY_PROPERTIES).flat_map do |field_name, field_mapping|
+            path = "#{parent_path}#{field_name}"
+            [path] + mapping_field_paths_of(field_mapping, parent_path: "#{path}.")
+          end
+        end
+
         def cannot_modify_mapping_field_type_error
           "The datastore does not support modifying the type of a field from an existing index definition. " \
           "You are attempting to update type of fields (#{mapping_type_changes.inspect}) from the #{@index_template.name} index definition."
@@ -92,10 +118,6 @@ def index_template_exists?
           !current_config_parent.empty?
         end
 
-        def mapping_removals
-          @mapping_removals ||= mapping_fields_from(current_mapping) - mapping_fields_from(desired_mapping)
-        end
-
         def mapping_type_changes
           @mapping_type_changes ||= begin
             flattened_current = Support::HashUtil.flatten_and_stringify_keys(current_mapping)
@@ -108,7 +130,7 @@ def mapping_type_changes
         end
 
         def has_mapping_updates?
-          current_mapping != desired_mapping
+          current_mapping != desired_mapping_for_update
         end
 
         def settings_updates
@@ -119,15 +141,19 @@ def settings_updates
           end
         end
 
-        def mapping_fields_from(mapping_hash, prefix = "")
-          (mapping_hash["properties"] || []).flat_map do |key, params|
-            field = prefix + key
-            if params.key?("properties")
-              [field] + mapping_fields_from(params, "#{field}.")
-            else
-              [field]
-            end
-          end
+        def desired_mapping_for_update
+          @desired_mapping_for_update ||= MappingUpdate.build_mapping_update(
+            desired: desired_mapping,
+            current: current_mapping,
+            protected_field_paths: @index_template.field_paths_protected_from_removal
+          )
+        end
+
+        def desired_config_parent_for_update
+          @desired_config_parent_for_update ||= Support::HashUtil.deep_merge(
+            desired_config_parent,
+            {"template" => {"mappings" => desired_mapping_for_update}}
+          )
         end
 
         def desired_mapping
@@ -178,43 +204,13 @@ def current_config_parent
         end
 
         def config_diff
-          @config_diff ||= Indexer::HashDiffer.diff(current_config_parent, desired_config_parent) || "(no diff)"
+          @config_diff ||= Indexer::HashDiffer.diff(current_config_parent, desired_config_parent_for_update) || "(no diff)"
         end
 
         def report_action(message)
           @reporter.report_action(message)
         end
 
-        # Helper method used to merge properties between a _desired_ configuration and a _current_ configuration.
-        # This is used when we are figuring out how to update an index template. We do not want to delete existing
-        # fields from a template--while the datastore would allow it, our schema evolution strategy depends upon
-        # us not dropping old unused fields. The datastore doesn't allow it on indices, anyway (though it does allow
-        # it on index templates). We've ran into trouble (a near SEV) when allowing the logic here to delete an unused
-        # field from an index template. The indexer "mapping completeness" check started failing because an old version
-        # of the code (from back when the field in question was still used) noticed the expected field was missing and
-        # started failing on every event.
-        #
-        # This helps us avoid that problem by retaining any currently existing fields.
-        #
-        # Long term, if we want to support fully "garbage collecting" these old fields on templates, we will need
-        # to have them get dropped in a follow up step. We could have our `update_datastore_config` script notice that
-        # the deployed prod indexers are at a version that will tolerate the fields being dropped, or support it
-        # via an opt-in flag or something.
-        def merge_properties(desired_object, current_object)
-          desired_properties = desired_object.fetch("properties") { _ = {} }
-          current_properties = current_object.fetch("properties") { _ = {} }
-
-          merged_properties = desired_properties.merge(current_properties) do |key, desired, current|
-            if current.is_a?(::Hash) && current.key?("properties") && desired.key?("properties")
-              merge_properties(desired, current)
-            else
-              desired
-            end
-          end
-
-          desired_object.merge("properties" => merged_properties)
-        end
-
         def related_index_configurators
           # Here we fan out and get a configurator for each related index. These are generally concrete
           # index that are based on a template, either via being specified in our config YAML, or via
diff --git a/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/mapping_update.rb b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/mapping_update.rb
new file mode 100644
index 000000000..fa8f68fa4
--- /dev/null
+++ b/elasticgraph-admin/lib/elastic_graph/admin/index_definition_configurator/mapping_update.rb
@@ -0,0 +1,61 @@
+# Copyright 2024 - 2026 Block, Inc.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+# frozen_string_literal: true
+
+module ElasticGraph
+  class Admin
+    module IndexDefinitionConfigurator
+      module MappingUpdate
+        # Steep's `UnannotatedEmptyCollection` diagnostic requires the annotation to be on the line
+        # containing the `{}` literal and cannot see it through `.freeze`, hence the two-step assignment.
+        empty_properties = {} # : ::Hash[::String, untyped]
+        EMPTY_PROPERTIES = empty_properties.freeze
+
+        # Elasticsearch/OpenSearch do not support removing mapping fields from an index. Preserve current
+        # fields when building index mapping update payloads and diffs, while still allowing updates to
+        # existing field parameters and additions of new fields.
+        #
+        # `protected_field_paths` limits which fields missing from the desired mapping get preserved: when
+        # provided, a missing field is preserved only if its path (or a descendant's path) is in the set,
+        # and other missing fields are dropped from the built update. When `nil`, all missing fields are
+        # preserved.
+        def self.build_mapping_update(desired:, current:, protected_field_paths: nil, parent_path: "")
+          desired_properties = desired.fetch("properties", EMPTY_PROPERTIES)
+          current_properties = current.fetch("properties", EMPTY_PROPERTIES)
+
+          preserved_current_properties = current_properties.select do |field_name, _|
+            desired_properties.key?(field_name) || preserve_missing_field?(protected_field_paths, "#{parent_path}#{field_name}")
+          end
+
+          merged_properties = desired_properties.merge(preserved_current_properties) do |field_name, desired_value, current_value|
+            if current_value.is_a?(::Hash) && current_value.key?("properties") && desired_value.key?("properties")
+              build_mapping_update(
+                desired: desired_value,
+                current: current_value,
+                protected_field_paths: protected_field_paths,
+                parent_path: "#{parent_path}#{field_name}."
+              )
+            else
+              desired_value
+            end
+          end
+
+          desired.merge("properties" => merged_properties)
+        end
+
+        # A missing field is preserved if it is protected itself, or if any protected path sits underneath
+        # it (dropping the field would drop the protected descendant along with it).
+        def self.preserve_missing_field?(protected_field_paths, path)
+          return true if protected_field_paths.nil?
+
+          protected_field_paths.include?(path) || protected_field_paths.any? { |protected_path| protected_path.start_with?("#{path}.") }
+        end
+        private_class_method :preserve_missing_field?
+      end
+    end
+  end
+end
diff --git a/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index.rbs b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index.rbs
index 54fbd0c4b..a2a49fa72 100644
--- a/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index.rbs
+++ b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index.rbs
@@ -28,9 +28,6 @@ module ElasticGraph
         def cannot_modify_mapping_field_type_error: () -> ::String
         def index_exists?: () -> bool
 
-        @mapping_removals: ::Array[::String]?
-        def mapping_removals: () -> ::Array[::String]
-
         @mapping_type_changes: ::Array[::String]?
         def mapping_type_changes: () -> ::Array[::String]
 
@@ -39,7 +36,8 @@ module ElasticGraph
         @settings_updates: DatastoreCore::indexSettingsHash?
         def settings_updates: () -> DatastoreCore::indexSettingsHash
 
-        def mapping_fields_from: (DatastoreCore::indexMappingHash, ?::String) -> ::Array[::String]
+        @desired_mapping_for_update: DatastoreCore::indexMappingHash?
+        def desired_mapping_for_update: () -> DatastoreCore::indexMappingHash
 
         def desired_mapping: () -> DatastoreCore::indexMappingHash
 
diff --git a/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index_template.rbs b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index_template.rbs
index 68943db03..14826d9d0 100644
--- a/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index_template.rbs
+++ b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/for_index_template.rbs
@@ -26,12 +26,11 @@ module ElasticGraph
         @clock: singleton(::Time)
 
         def put_index_template: () -> void
+        def removed_fields_note: () -> ::String
+        def mapping_field_paths_of: (DatastoreCore::indexMappingHash, ?parent_path: ::String) -> ::Array[::String]
         def cannot_modify_mapping_field_type_error: () -> ::String
         def index_template_exists?: () -> bool
 
-        @mapping_removals: ::Array[::String]?
-        def mapping_removals: () -> ::Array[::String]
-
         @mapping_type_changes: ::Array[::String]?
         def mapping_type_changes: () -> ::Array[::String]
 
@@ -40,7 +39,11 @@ module ElasticGraph
         @settings_updates: DatastoreCore::indexSettingsHash?
         def settings_updates: () -> DatastoreCore::indexSettingsHash
 
-        def mapping_fields_from: (DatastoreCore::indexMappingHash, ?::String) -> ::Array[::String]
+        @desired_mapping_for_update: DatastoreCore::indexMappingHash?
+        def desired_mapping_for_update: () -> DatastoreCore::indexMappingHash
+
+        @desired_config_parent_for_update: ::Hash[::String, untyped]?
+        def desired_config_parent_for_update: () -> ::Hash[::String, untyped]
 
         def desired_mapping: () -> DatastoreCore::indexMappingHash
 
@@ -62,7 +65,6 @@ module ElasticGraph
         def config_diff: () -> ::String
 
         def report_action: (::String) -> void
-        def merge_properties: (::Hash[::String, untyped], ::Hash[::String, untyped]) -> ::Hash[::String, untyped]
 
         @related_index_configurators: ::Array[ForIndex]?
         def related_index_configurators: () -> ::Array[ForIndex]
diff --git a/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/mapping_update.rbs b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/mapping_update.rbs
new file mode 100644
index 000000000..1e3e36207
--- /dev/null
+++ b/elasticgraph-admin/sig/elastic_graph/admin/index_definition_configurator/mapping_update.rbs
@@ -0,0 +1,18 @@
+module ElasticGraph
+  class Admin
+    module IndexDefinitionConfigurator
+      module MappingUpdate
+        EMPTY_PROPERTIES: ::Hash[::String, untyped]
+
+        def self.build_mapping_update: (
+          desired: ::Hash[::String, untyped],
+          current: ::Hash[::String, untyped],
+          ?protected_field_paths: ::Set[::String]?,
+          ?parent_path: ::String
+        ) -> ::Hash[::String, untyped]
+
+        def self.preserve_missing_field?: (::Set[::String]?, ::String) -> bool
+      end
+    end
+  end
+end
diff --git a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_spec.rb b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_spec.rb
index 113f36276..dc7e215cd 100644
--- a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_spec.rb
+++ b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_spec.rb
@@ -35,6 +35,27 @@ module IndexDefinitionConfigurator
             }.to make_no_datastore_write_calls("main")
           end
 
+          # Note: this behavior differs from index templates (see `for_index_template_spec.rb`): the datastore
+          # allows template fields to be dropped, but provides no way to remove fields from a concrete index.
+          it "is a no-op when attempting to drop a mapping field because the datastore does not support it" do
+            configure_index_definition(schema_def)
+            output_io.string = +"" # use `+` so it is not a frozen string literal.
+
+            expect {
+              # Here we remove the `name` field and the `options.size` field to verify it works for both root and nested fields.
+              configure_index_definition(schema_def(
+                avoid_defining_widget_fields: %w[name],
+                avoid_defining_widget_options_fields: %w[size]
+              ))
+            }.to maintain {
+              props = get_index_definition_configuration(unique_index_name).dig("mappings", "properties")
+              [props.keys.sort, props.dig("options", "properties").keys.sort]
+            }.from([[*index_meta_fields, "created_at", "id", "name", "options"], ["color", "size"]])
+              .and make_no_datastore_write_calls("main")
+
+            expect(output_io.string).to exclude("Updated mappings", "properties.name", "properties.options.properties.size")
+          end
+
           def make_datastore_calls_to_configure_index_def(index_name, subresource = nil)
             make_datastore_write_calls("main", "PUT #{put_index_definition_url(index_name, subresource)}")
           end
diff --git a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_template_spec.rb b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_template_spec.rb
index ef04c9cd9..faf70dab2 100644
--- a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_template_spec.rb
+++ b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/for_index_template_spec.rb
@@ -95,6 +95,56 @@ def fetch_artifact_configuration(schema_artifacts, index_def_name)
               .and make_datastore_calls_to_configure_index_def(unique_index_name, :settings)
           end
 
+          it "drops mapping fields whose last schema reference has been removed, while leaving concrete indices unchanged" do
+            configure_index_definition(schema_def)
+            output_io.string = +"" # use `+` so it is not a frozen string literal.
+
+            expect {
+              # Here we remove the `name` field and the `options.size` field to verify it works for both root and nested fields.
+              configure_index_definition(schema_def(
+                avoid_defining_widget_fields: %w[name],
+                avoid_defining_widget_options_fields: %w[size]
+              ))
+            }.to change {
+              props = get_index_template_definition_configuration(unique_index_name).dig("mappings", "properties")
+              [props.keys.sort, props.dig("options", "properties").keys.sort]
+            }.from([[*index_meta_fields, "created_at", "id", "name", "options"], ["color", "size"]])
+              .to([[*index_meta_fields, "created_at", "id", "options"], ["color"]])
+              .and maintain {
+                props = main_datastore_client.get_index(concrete_index_name_for_now(unique_index_name)).dig("mappings", "properties")
+                [props.keys.sort, props.dig("options", "properties").keys.sort]
+              }.from([[*index_meta_fields, "created_at", "id", "name", "options"], ["color", "size"]])
+              .and make_datastore_write_calls("main", "PUT #{put_index_template_definition_url(unique_index_name)}")
+
+            expect(output_io.string).to include(
+              "Updated index template: `#{unique_index_name}`",
+              "properties.name", "properties.options.properties.size",
+              "Removed 2 stale field(s) from index template `#{unique_index_name}` that are no longer referenced by the schema: name, options.size.",
+              "New rollover indices created from this template will reject documents that still contain these fields (mappings are `dynamic: strict`)."
+            )
+          end
+
+          it "preserves removed mapping fields that `deleted_field` declarations still reference, since indexers running an older schema version may still write them" do
+            configure_index_definition(schema_def)
+            output_io.string = +"" # use `+` so it is not a frozen string literal.
+
+            expect {
+              # Here we remove the `name` field and the `options.size` field to verify it works for both root and nested fields.
+              configure_index_definition(schema_def(
+                avoid_defining_widget_fields: %w[name],
+                avoid_defining_widget_options_fields: %w[size],
+                configure_widget: ->(t) { t.deleted_field "name" },
+                configure_widget_options: ->(t) { t.deleted_field "size" }
+              ))
+            }.to maintain {
+              props = get_index_template_definition_configuration(unique_index_name).dig("mappings", "properties")
+              [props.keys.sort, props.dig("options", "properties").keys.sort]
+            }.from([[*index_meta_fields, "created_at", "id", "name", "options"], ["color", "size"]])
+              .and make_no_datastore_write_calls("main")
+
+            expect(output_io.string).to exclude("Updated index template", "Removed")
+          end
+
           it "creates concrete indices based on `setting_overrides_by_timestamp` configuration, and avoids creating an extra index for 'now'" do
             jan_2020_index_name = unique_index_name + "_rollover__2020-01"
 
diff --git a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/shared_examples.rb b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/shared_examples.rb
index 0ee07c00a..8783e6faf 100644
--- a/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/shared_examples.rb
+++ b/elasticgraph-admin/spec/integration/elastic_graph/admin/index_definition_configurator/shared_examples.rb
@@ -41,7 +41,6 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
       RSpec.shared_examples_for IndexDefinitionConfigurator, :uses_datastore do
         let(:output_io) { StringIO.new }
         let(:clock) { class_double(::Time, now: ::Time.utc(2024, 3, 20, 12, 0, 0)) }
-        let(:mapping_removal_note_snippet) { "extra fields listed here will not actually get removed" }
         let(:index_meta_fields) { ["__nested_sourced_data", "__sources", "__typename", "__versions"] }
 
         it "idempotently creates an index or index template, avoiding unneeded datastore write calls" do
@@ -60,7 +59,7 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
           }.to maintain { get_index_definition_configuration(unique_index_name) }
             .and make_no_datastore_write_calls("main")
 
-          expect(output_io.string).not_to include(mapping_removal_note_snippet)
+          expect(output_io.string).to exclude("+ mappings", "+ settings")
         end
 
         it "allows new top-level fields to be added to an existing index or index template" do
@@ -77,7 +76,6 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
           # The printed description of what was changed should not mention settings that are not being updated.
           # (Requires us to normalize settings properly in the logic for this to be the case).
           expect(output_io.string).to include("properties.amount_cents").and exclude("coerce", "ignore_malformed", "number_of_replicas", "number_of_shards")
-          expect(output_io.string).not_to include(mapping_removal_note_snippet)
         end
 
         it "handles both `object` lists and `nested` lists" do
@@ -153,6 +151,7 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
               end
             }
           ))
+          output_io.string = +"" # use `+` so it is not a frozen string literal.
 
           expect {
             configure_index_definition(schema_def)
@@ -160,6 +159,8 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
             .from({"type" => "keyword", "meta" => {"foo" => "1"}})
             .to({"type" => "keyword"})
             .and make_datastore_calls_to_configure_index_def(unique_index_name, :mappings)
+
+          expect(output_io.string).to include("properties.name.meta")
         end
 
         it "allows some previously unset settings to be changed on an existing index or index template" do
@@ -202,24 +203,6 @@ def simulate_presence_of_extra_setting(admin, index_definition_name, name, value
           }.to make_no_datastore_write_calls("main")
         end
 
-        it "is a no-op when we attempt to drop a field because the datastore doesn't support dropping mapping fields" do
-          configure_index_definition(schema_def)
-
-          expect {
-            # Here we remove the `name` field and the `options.size` field to verify it works for both root and nested fields.
-            configure_index_definition(schema_def(
-              avoid_defining_widget_fields: %w[name],
-              avoid_defining_widget_options_fields: %w[size]
-            ))
-          }.to maintain {
-            props = get_index_definition_configuration(unique_index_name).dig("mappings", "properties")
-            [props.keys.sort, props.dig("options", "properties").keys.sort]
-          }.from([[*index_meta_fields, "created_at", "id", "name", "options"], ["color", "size"]])
-            .and make_datastore_calls_to_configure_index_def(unique_index_name, :mappings)
-
-          expect(output_io.string).to include(mapping_removal_note_snippet)
-        end
-
         it "maintains `_meta.ElasticGraph.sources` as a stateful append-only-set that remembers sources that were once active but we no longer have" do
           expect {
             configure_index_definition(schema_def(
diff --git a/elasticgraph-admin/spec/unit/elastic_graph/admin/index_definition_configurator/mapping_update_spec.rb b/elasticgraph-admin/spec/unit/elastic_graph/admin/index_definition_configurator/mapping_update_spec.rb
new file mode 100644
index 000000000..412d90617
--- /dev/null
+++ b/elasticgraph-admin/spec/unit/elastic_graph/admin/index_definition_configurator/mapping_update_spec.rb
@@ -0,0 +1,154 @@
+# Copyright 2024 - 2026 Block, Inc.
+#
+# Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+#
+# frozen_string_literal: true
+
+require "elastic_graph/admin/index_definition_configurator/mapping_update"
+
+module ElasticGraph
+  class Admin
+    module IndexDefinitionConfigurator
+      RSpec.describe MappingUpdate do
+        describe ".build_mapping_update" do
+          it "preserves current mapping fields that are missing from the desired mapping, at both root and nested levels" do
+            current = {
+              "properties" => {
+                "name" => {"type" => "keyword"},
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"},
+                    "size" => {"type" => "keyword"}
+                  }
+                }
+              }
+            }
+
+            desired = {
+              "properties" => {
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"},
+                    "weight" => {"type" => "integer"}
+                  }
+                }
+              }
+            }
+
+            expect(described_class.build_mapping_update(desired: desired, current: current)).to eq({
+              "properties" => {
+                "name" => {"type" => "keyword"},
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"},
+                    "weight" => {"type" => "integer"},
+                    "size" => {"type" => "keyword"}
+                  }
+                }
+              }
+            })
+          end
+
+          it "drops current-only fields that are not protected when `protected_field_paths` is provided, while preserving protected ones" do
+            current = {
+              "properties" => {
+                "name" => {"type" => "keyword"},
+                "legacy_field" => {"type" => "keyword"},
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"},
+                    "size" => {"type" => "keyword"},
+                    "weight" => {"type" => "integer"}
+                  }
+                }
+              }
+            }
+
+            desired = {
+              "properties" => {
+                "name" => {"type" => "keyword"},
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"}
+                  }
+                }
+              }
+            }
+
+            result = described_class.build_mapping_update(
+              desired: desired,
+              current: current,
+              protected_field_paths: ["options.size"].to_set
+            )
+
+            expect(result).to eq({
+              "properties" => {
+                "name" => {"type" => "keyword"},
+                "options" => {
+                  "properties" => {
+                    "color" => {"type" => "keyword"},
+                    "size" => {"type" => "keyword"}
+                  }
+                }
+              }
+            })
+          end
+
+          it "preserves an entire current-only parent field when a protected path sits underneath it, since dropping the parent would drop the protected field" do
+            current = {
+              "properties" => {
+                "old_parent" => {
+                  "properties" => {
+                    "keep_me" => {"type" => "keyword"},
+                    "sibling" => {"type" => "keyword"}
+                  }
+                }
+              }
+            }
+
+            desired = {"properties" => {}}
+
+            result = described_class.build_mapping_update(
+              desired: desired,
+              current: current,
+              protected_field_paths: ["old_parent.keep_me"].to_set
+            )
+
+            expect(result).to eq({
+              "properties" => {
+                "old_parent" => {
+                  "properties" => {
+                    "keep_me" => {"type" => "keyword"},
+                    "sibling" => {"type" => "keyword"}
+                  }
+                }
+              }
+            })
+          end
+
+          it "favors the desired mapping for fields present in both, allowing existing field parameters to be removed" do
+            current = {
+              "properties" => {
+                "name" => {"type" => "keyword", "meta" => {"foo" => "1"}}
+              }
+            }
+
+            desired = {
+              "properties" => {
+                "name" => {"type" => "keyword"}
+              }
+            }
+
+            expect(described_class.build_mapping_update(desired: desired, current: current)).to eq({
+              "properties" => {
+                "name" => {"type" => "keyword"}
+              }
+            })
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition.rb b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition.rb
index 94557efcd..7fb5ae8d7 100644
--- a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition.rb
+++ b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition.rb
@@ -30,6 +30,7 @@ def self.with(name:, runtime_metadata:, config:, datastore_clients_by_name:, sch
           default_sort_clauses: runtime_metadata.default_sort_fields.map(&:to_query_clause),
           current_sources: runtime_metadata.current_sources,
           fields_by_path: runtime_metadata.fields_by_path,
+          field_paths_protected_from_removal: runtime_metadata.field_paths_protected_from_removal,
           env_index_config: env_index_config,
           defined_clusters: config.clusters.keys.to_set,
           datastore_clients_by_name: datastore_clients_by_name,
diff --git a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/index.rb b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/index.rb
index 3d4ad6d30..cf83d7544 100644
--- a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/index.rb
+++ b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/index.rb
@@ -14,14 +14,14 @@ module ElasticGraph
   class DatastoreCore
     module IndexDefinition
       class Index < Support::MemoizableData.define(
-        :name, :route_with, :default_sort_clauses, :current_sources, :fields_by_path,
+        :name, :route_with, :default_sort_clauses, :current_sources, :fields_by_path, :field_paths_protected_from_removal,
         :env_index_config, :defined_clusters, :datastore_clients_by_name, :env_agnostic_settings, :has_had_multiple_sources,
         :sourced_from_nested_paths_by_qualified_relationship
       )
         # `Data.define` provides all these methods:
         # @dynamic name, route_with, default_sort_clauses, current_sources, fields_by_path, env_index_config, env_agnostic_settings
         # @dynamic defined_clusters, datastore_clients_by_name, initialize, has_had_multiple_sources
-        # @dynamic sourced_from_nested_paths_by_qualified_relationship
+        # @dynamic sourced_from_nested_paths_by_qualified_relationship, field_paths_protected_from_removal
 
         # `include IndexDefinition::Base` provides all these methods. Steep should be able to detect it
         # but can't for some reason so we have to declare them with `@dynamic`.
diff --git a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/rollover_index_template.rb b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/rollover_index_template.rb
index 4b0ba4b42..a1f35df70 100644
--- a/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/rollover_index_template.rb
+++ b/elasticgraph-datastore_core/lib/elastic_graph/datastore_core/index_definition/rollover_index_template.rb
@@ -21,14 +21,14 @@ module ElasticGraph
   class DatastoreCore
     module IndexDefinition
       class RolloverIndexTemplate < Support::MemoizableData.define(
-        :name, :route_with, :default_sort_clauses, :current_sources, :fields_by_path, :env_index_config,
+        :name, :route_with, :default_sort_clauses, :current_sources, :fields_by_path, :field_paths_protected_from_removal, :env_index_config,
         :index_args, :defined_clusters, :datastore_clients_by_name, :timestamp_field_path, :frequency,
         :env_agnostic_settings, :has_had_multiple_sources, :sourced_from_nested_paths_by_qualified_relationship
       )
         # `Data.define` provides all these methods:
         # @dynamic name, route_with, default_sort_clauses, current_sources, fields_by_path, env_index_config, env_agnostic_settings
         # @dynamic index_args, defined_clusters, datastore_clients_by_name, timestamp_field_path, frequency, initialize, has_had_multiple_sources
-        # @dynamic sourced_from_nested_paths_by_qualified_relationship
+        # @dynamic sourced_from_nested_paths_by_qualified_relationship, field_paths_protected_from_removal
 
         # `include IndexDefinition::Base` provides all these methods. Steep should be able to detect it
         # but can't for some reason so we have to declare them with `@dynamic`.
diff --git a/elasticgraph-datastore_core/sig/elastic_graph/datastore_core/index_definition.rbs b/elasticgraph-datastore_core/sig/elastic_graph/datastore_core/index_definition.rbs
index b82b6a313..61964c24d 100644
--- a/elasticgraph-datastore_core/sig/elastic_graph/datastore_core/index_definition.rbs
+++ b/elasticgraph-datastore_core/sig/elastic_graph/datastore_core/index_definition.rbs
@@ -26,6 +26,7 @@ module ElasticGraph
       def default_sort_clauses: () -> ::Array[::Hash[::String, ::String]]
       def current_sources: () -> ::Set[::String]
       def fields_by_path: () -> ::Hash[::String, SchemaArtifacts::RuntimeMetadata::IndexField]
+      def field_paths_protected_from_removal: () -> ::Set[::String]
       def has_had_multiple_sources: () -> bool
       def sourced_from_nested_paths_by_qualified_relationship: () -> ::Hash[::String, ::Array[SchemaArtifacts::RuntimeMetadata::sourcedFromNestedPathSegment]]
       def env_index_config: () -> Configuration::IndexDefinition
diff --git a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/index_spec.rb b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/index_spec.rb
index c80f07c9a..bafd7dc80 100644
--- a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/index_spec.rb
+++ b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/index_spec.rb
@@ -88,6 +88,7 @@ def index_def_named(name, rollover: nil)
             default_sort_fields: [],
             current_sources: [SELF_RELATIONSHIP_NAME],
             fields_by_path: {},
+            field_paths_protected_from_removal: [],
             has_had_multiple_sources: false,
             sourced_from_nested_paths_by_qualified_relationship: {}
           )
diff --git a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
index f7d05161b..42b08bec9 100644
--- a/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
+++ b/elasticgraph-datastore_core/spec/integration/elastic_graph/datastore_core/index_definition/rollover_index_template_spec.rb
@@ -557,6 +557,7 @@ def index_def_named(name, rollover: nil)
             default_sort_fields: [],
             current_sources: [SELF_RELATIONSHIP_NAME],
             fields_by_path: {},
+            field_paths_protected_from_removal: [],
             has_had_multiple_sources: false,
             sourced_from_nested_paths_by_qualified_relationship: {}
           )
diff --git a/elasticgraph-local/spec/acceptance/rake_tasks_spec.rb b/elasticgraph-local/spec/acceptance/rake_tasks_spec.rb
index 3c8d60519..1b2be1f64 100644
--- a/elasticgraph-local/spec/acceptance/rake_tasks_spec.rb
+++ b/elasticgraph-local/spec/acceptance/rake_tasks_spec.rb
@@ -33,7 +33,7 @@ module Local
               # ...dumps schema artifacts...
               "datastore_config.yaml` is already up to date",
               # ...configures Elasticsearch...
-              "Updated index template: `widgets`",
+              "Created index template: `widgets`",
               # ...indexes a document...
               "Published batch of 1 document"
             )
diff --git a/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rb b/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rb
index df542e18d..7057a73ae 100644
--- a/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rb
+++ b/elasticgraph-schema_artifacts/lib/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rb
@@ -17,22 +17,24 @@ module RuntimeMetadata
       # Runtime metadata related to a datastore index definition.
       #
       # @private
-      class IndexDefinition < ::Data.define(:route_with, :rollover, :default_sort_fields, :current_sources, :fields_by_path, :has_had_multiple_sources, :sourced_from_nested_paths_by_qualified_relationship)
+      class IndexDefinition < ::Data.define(:route_with, :rollover, :default_sort_fields, :current_sources, :fields_by_path, :field_paths_protected_from_removal, :has_had_multiple_sources, :sourced_from_nested_paths_by_qualified_relationship)
         ROUTE_WITH = "route_with"
         ROLLOVER = "rollover"
         DEFAULT_SORT_FIELDS = "default_sort_fields"
         CURRENT_SOURCES = "current_sources"
         FIELDS_BY_PATH = "fields_by_path"
+        FIELD_PATHS_PROTECTED_FROM_REMOVAL = "field_paths_protected_from_removal"
         HAS_HAD_MULTIPLE_SOURCES = "has_had_multiple_sources"
         SOURCED_FROM_NESTED_PATHS_BY_QUALIFIED_RELATIONSHIP = "sourced_from_nested_paths_by_qualified_relationship"
 
-        def initialize(route_with:, rollover:, default_sort_fields:, current_sources:, fields_by_path:, has_had_multiple_sources:, sourced_from_nested_paths_by_qualified_relationship:)
+        def initialize(route_with:, rollover:, default_sort_fields:, current_sources:, fields_by_path:, field_paths_protected_from_removal:, has_had_multiple_sources:, sourced_from_nested_paths_by_qualified_relationship:)
           super(
             route_with: route_with,
             rollover: rollover,
             default_sort_fields: default_sort_fields,
             current_sources: current_sources.to_set,
             fields_by_path: fields_by_path,
+            field_paths_protected_from_removal: field_paths_protected_from_removal.to_set,
             has_had_multiple_sources: has_had_multiple_sources,
             sourced_from_nested_paths_by_qualified_relationship: sourced_from_nested_paths_by_qualified_relationship
           )
@@ -45,6 +47,7 @@ def self.from_hash(hash)
             default_sort_fields: hash[DEFAULT_SORT_FIELDS]&.map { |h| SortField.from_hash(h) } || [],
             current_sources: hash[CURRENT_SOURCES] || [],
             fields_by_path: (hash[FIELDS_BY_PATH] || {}).transform_values { |h| IndexField.from_hash(h) },
+            field_paths_protected_from_removal: hash[FIELD_PATHS_PROTECTED_FROM_REMOVAL] || [],
             has_had_multiple_sources: hash[HAS_HAD_MULTIPLE_SOURCES] || false,
             sourced_from_nested_paths_by_qualified_relationship: (hash[SOURCED_FROM_NESTED_PATHS_BY_QUALIFIED_RELATIONSHIP] || {}).transform_values { |segments| segments.map { |h| SourcedFromNestedPathSegment.from_hash(h) } }
           )
@@ -56,6 +59,7 @@ def to_dumpable_hash
             CURRENT_SOURCES => current_sources.sort,
             DEFAULT_SORT_FIELDS => default_sort_fields.map(&:to_dumpable_hash),
             FIELDS_BY_PATH => HashDumper.dump_hash(fields_by_path, &:to_dumpable_hash),
+            FIELD_PATHS_PROTECTED_FROM_REMOVAL => field_paths_protected_from_removal.sort,
             HAS_HAD_MULTIPLE_SOURCES => (true if has_had_multiple_sources),
             ROLLOVER => rollover&.to_dumpable_hash,
             ROUTE_WITH => route_with,
diff --git a/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rbs b/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rbs
index 645309b9d..a174af695 100644
--- a/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rbs
+++ b/elasticgraph-schema_artifacts/sig/elastic_graph/schema_artifacts/runtime_metadata/index_definition.rbs
@@ -7,6 +7,7 @@ module ElasticGraph
         attr_reader default_sort_fields: ::Array[SortField]
         attr_reader current_sources: ::Set[::String]
         attr_reader fields_by_path: ::Hash[::String, IndexField]
+        attr_reader field_paths_protected_from_removal: ::Set[::String]
         attr_reader has_had_multiple_sources: bool
         attr_reader sourced_from_nested_paths_by_qualified_relationship: ::Hash[::String, ::Array[sourcedFromNestedPathSegment]]
 
@@ -16,6 +17,7 @@ module ElasticGraph
           default_sort_fields: ::Array[SortField],
           current_sources: ::Set[::String],
           fields_by_path: ::Hash[::String, IndexField],
+          field_paths_protected_from_removal: ::Set[::String],
           has_had_multiple_sources: bool,
           sourced_from_nested_paths_by_qualified_relationship: ::Hash[::String, ::Array[sourcedFromNestedPathSegment]]
         ) -> void
@@ -26,6 +28,7 @@ module ElasticGraph
           ?default_sort_fields: ::Array[SortField],
           ?current_sources: ::Enumerable[::String],
           ?fields_by_path: ::Hash[::String, IndexField],
+          ?field_paths_protected_from_removal: ::Enumerable[::String],
           ?has_had_multiple_sources: bool,
           ?sourced_from_nested_paths_by_qualified_relationship: ::Hash[::String, ::Array[sourcedFromNestedPathSegment]]
         ) -> IndexDefinition
@@ -37,6 +40,7 @@ module ElasticGraph
         DEFAULT_SORT_FIELDS: "default_sort_fields"
         CURRENT_SOURCES: "current_sources"
         FIELDS_BY_PATH: "fields_by_path"
+        FIELD_PATHS_PROTECTED_FROM_REMOVAL: "field_paths_protected_from_removal"
         HAS_HAD_MULTIPLE_SOURCES: "has_had_multiple_sources"
         SOURCED_FROM_NESTED_PATHS_BY_QUALIFIED_RELATIONSHIP: "sourced_from_nested_paths_by_qualified_relationship"
 
@@ -46,6 +50,7 @@ module ElasticGraph
           default_sort_fields: ::Array[SortField],
           current_sources: ::Enumerable[::String],
           fields_by_path: ::Hash[::String, IndexField],
+          field_paths_protected_from_removal: ::Enumerable[::String],
           has_had_multiple_sources: bool,
           sourced_from_nested_paths_by_qualified_relationship: ::Hash[::String, ::Array[sourcedFromNestedPathSegment]]
         ) -> void
diff --git a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/index_definition_spec.rb b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/index_definition_spec.rb
index 0939b9fb3..1801601cf 100644
--- a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/index_definition_spec.rb
+++ b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/index_definition_spec.rb
@@ -24,6 +24,7 @@ module RuntimeMetadata
             default_sort_fields: [],
             current_sources: Set.new,
             fields_by_path: {},
+            field_paths_protected_from_removal: Set.new,
             has_had_multiple_sources: false,
             sourced_from_nested_paths_by_qualified_relationship: {}
           )
@@ -44,6 +45,14 @@ module RuntimeMetadata
           })
         end
 
+        it "round-trips `field_paths_protected_from_removal` through a dumped hash as a sorted list" do
+          index_def = index_definition_with(field_paths_protected_from_removal: ["options.size", "name"])
+
+          dumped = index_def.to_dumpable_hash
+          expect(dumped["field_paths_protected_from_removal"]).to eq ["name", "options.size"]
+          expect(IndexDefinition.from_hash(dumped).field_paths_protected_from_removal).to eq ["name", "options.size"].to_set
+        end
+
         it "prunes `has_had_multiple_sources: false` from the dumped hash but includes `has_had_multiple_sources: true`" do
           index_def_without_flag = index_definition_with(has_had_multiple_sources: false)
           expect(index_def_without_flag.to_dumpable_hash["has_had_multiple_sources"]).to be_nil
diff --git a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_spec.rb b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_spec.rb
index 058254673..3909a6e65 100644
--- a/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_spec.rb
+++ b/elasticgraph-schema_artifacts/spec/unit/elastic_graph/schema_artifacts/runtime_metadata/schema_spec.rb
@@ -127,6 +127,7 @@ module RuntimeMetadata
                 fields_by_path: {
                   "foo.bar" => IndexField.new(source: "other")
                 },
+                field_paths_protected_from_removal: ["foo.legacy"],
                 has_had_multiple_sources: false,
                 sourced_from_nested_paths_by_qualified_relationship: {
                   "currency" => [
@@ -141,6 +142,7 @@ module RuntimeMetadata
                 default_sort_fields: [],
                 current_sources: [SELF_RELATIONSHIP_NAME],
                 fields_by_path: {},
+                field_paths_protected_from_removal: [],
                 has_had_multiple_sources: false,
                 sourced_from_nested_paths_by_qualified_relationship: {}
               ),
@@ -150,6 +152,7 @@ module RuntimeMetadata
                 default_sort_fields: [],
                 current_sources: [SELF_RELATIONSHIP_NAME],
                 fields_by_path: {},
+                field_paths_protected_from_removal: [],
                 has_had_multiple_sources: true,
                 sourced_from_nested_paths_by_qualified_relationship: {}
               )
@@ -280,6 +283,7 @@ module RuntimeMetadata
                     "source" => "other"
                   }
                 },
+                "field_paths_protected_from_removal" => ["foo.legacy"],
                 "sourced_from_nested_paths_by_qualified_relationship" => {
                   "currency" => [
                     {"field" => "costs", "source_field" => "cost_id"},
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/indexing/index.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/indexing/index.rb
index 460d46789..ccb9b7b4c 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/indexing/index.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/indexing/index.rb
@@ -267,6 +267,7 @@ def runtime_metadata
             rollover: rollover_config&.runtime_metadata,
             current_sources: indexed_type.current_sources,
             fields_by_path: indexed_type.index_field_runtime_metadata_tuples.to_h,
+            field_paths_protected_from_removal: indexed_type.protected_from_removal_field_paths.uniq,
             default_sort_fields: default_sort_pairs.each_slice(2).map do |(graphql_field_path_name, direction)|
               SchemaArtifacts::RuntimeMetadata::SortField.new(
                 field_path: public_field_path(graphql_field_path_name, explanation: "it is referenced as an index `default_sort` field").path_in_index,
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/has_subtypes.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/has_subtypes.rb
index 7c0b200a7..a01820118 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/has_subtypes.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/mixins/has_subtypes.rb
@@ -77,6 +77,12 @@ def index_field_runtime_metadata_tuples(
           end
         end
 
+        def protected_from_removal_field_paths(path_prefix: "")
+          resolve_subtypes.flat_map do |t|
+            t.protected_from_removal_field_paths(path_prefix: path_prefix)
+          end
+        end
+
         private
 
         def merge_fields_by_name_from_subtypes
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/object_type.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/object_type.rb
index 057d27394..ebaad5271 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/object_type.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/object_type.rb
@@ -26,7 +26,7 @@ module SchemaElements
       #   end
       class ObjectType < DelegateClass(TypeWithSubfields)
         # DelegateClass(TypeWithSubfields) provides the following methods:
-        # @dynamic name, type_ref, to_sdl, derived_graphql_types, to_indexing_field_type, current_sources, index_field_runtime_metadata_tuples, graphql_only?, relay_pagination_type, relationships_by_name
+        # @dynamic name, type_ref, to_sdl, derived_graphql_types, to_indexing_field_type, current_sources, index_field_runtime_metadata_tuples, protected_from_removal_field_paths, graphql_only?, relay_pagination_type, relationships_by_name
         include Mixins::SupportsFilteringAndAggregation
 
         # `include HasIndices` provides the following methods:
diff --git a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/type_with_subfields.rb b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/type_with_subfields.rb
index dfc7f5e90..bad5ae5dd 100644
--- a/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/type_with_subfields.rb
+++ b/elasticgraph-schema_definition/lib/elastic_graph/schema_definition/schema_elements/type_with_subfields.rb
@@ -544,6 +544,35 @@ def index_field_runtime_metadata_tuples(
           end
         end
 
+        # Index field paths that no longer exist in the current schema but must be preserved on the index
+        # template mapping: as long as a `deleted_field` or `renamed_from` declaration references a field,
+        # an old JSON schema version may still reference it, meaning an indexer deployed with an older
+        # schema version may still write it. Note that the old field's `name_in_index` is not recorded
+        # anywhere, so we use the declared field name here--they are the same unless the old field used a
+        # custom `name_in_index`.
+        #
+        # @private
+        def protected_from_removal_field_paths(path_prefix: "")
+          deprecated_fields_by_old_name =
+            schema_def_state.deleted_fields_by_type_name_and_old_field_name.fetch(name) do
+              {} # : ::Hash[::String, DeprecatedElement]
+            end.merge(schema_def_state.renamed_fields_by_type_name_and_old_field_name.fetch(name) do
+              {} # : ::Hash[::String, DeprecatedElement]
+            end)
+
+          own_paths = deprecated_fields_by_old_name.keys.map { |field_name| path_prefix + field_name }
+
+          nested_paths = indexing_fields_by_name_in_index.flat_map do |name_in_index, field|
+            if (object_type = field.type.fully_unwrapped.as_object_type)
+              object_type.protected_from_removal_field_paths(path_prefix: "#{path_prefix}#{name_in_index}.")
+            else
+              [] # : ::Array[::String]
+            end
+          end
+
+          own_paths + nested_paths
+        end
+
         private
 
         def fields_sdl(&arg_selector)
diff --git a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/api.rbs b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/api.rbs
index c0e3b506e..07fae09ba 100644
--- a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/api.rbs
+++ b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/api.rbs
@@ -35,6 +35,7 @@ module ElasticGraph
           ?parent_source: ::String,
           ?list_counts_state: SchemaElements::ListCountsState
         ) -> ::Array[[::String, SchemaArtifacts::RuntimeMetadata::IndexField]]
+      def protected_from_removal_field_paths: (?path_prefix: ::String) -> ::Array[::String]
       def derived_indexed_types: () -> ::Array[Indexing::DerivedIndexedType]
       def verify_graphql_correctness!: () -> void
       def relay_pagination_type: () -> bool
diff --git a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/mixins/has_subtypes.rbs b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/mixins/has_subtypes.rbs
index 737825d1e..8ca15c6db 100644
--- a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/mixins/has_subtypes.rbs
+++ b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/mixins/has_subtypes.rbs
@@ -20,6 +20,8 @@ module ElasticGraph
           ?list_counts_state: SchemaElements::ListCountsState
         ) -> ::Array[[::String, SchemaArtifacts::RuntimeMetadata::IndexField]]
 
+        def protected_from_removal_field_paths: (?path_prefix: ::String) -> ::Array[::String]
+
         private
 
         def merge_fields_by_name_from_subtypes: () {
diff --git a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/schema_elements/type_with_subfields.rbs b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/schema_elements/type_with_subfields.rbs
index a5a576f94..7fe6e512f 100644
--- a/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/schema_elements/type_with_subfields.rbs
+++ b/elasticgraph-schema_definition/sig/elastic_graph/schema_definition/schema_elements/type_with_subfields.rbs
@@ -87,6 +87,8 @@ module ElasticGraph
           ?list_counts_state: ListCountsState
         ) -> ::Array[[::String, SchemaArtifacts::RuntimeMetadata::IndexField]]
 
+        def protected_from_removal_field_paths: (?path_prefix: ::String) -> ::Array[::String]
+
         private
 
         def fields_sdl: () ?{ (Field::argument) -> boolish } -> String
diff --git a/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/runtime_metadata/index_definitions_by_name_spec.rb b/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/runtime_metadata/index_definitions_by_name_spec.rb
index 180b1261d..3262ca2a3 100644
--- a/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/runtime_metadata/index_definitions_by_name_spec.rb
+++ b/elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/runtime_metadata/index_definitions_by_name_spec.rb
@@ -1193,6 +1193,65 @@ def sourced_from_nested_paths_for_index(index_name, &schema)
         end
       end
 
+      describe "`field_paths_protected_from_removal`" do
+        it "is empty when the schema definition has no `deleted_field` or `renamed_from` declarations" do
+          widgets = index_definition_metadata_for("widgets")
+
+          expect(widgets.field_paths_protected_from_removal).to be_empty
+        end
+
+        it "includes fields referenced by `deleted_field` and `renamed_from` declarations on the indexed type" do
+          widgets = index_definition_metadata_for("widgets", on_my_type: ->(t) {
+            t.deleted_field "old_field"
+            t.field "new_name", "String" do |f|
+              f.renamed_from "old_name"
+            end
+          })
+
+          expect(widgets.field_paths_protected_from_removal).to eq ["old_field", "old_name"].to_set
+        end
+
+        it "includes declarations on embedded object types at their full path, using the `name_in_index` of parent fields" do
+          runtime_metadata = define_schema do |s|
+            s.object_type "NestedFields" do |t|
+              t.field "some_id_gql", "ID", name_in_index: "some_id_index"
+              t.deleted_field "old_nested_field"
+            end
+
+            s.object_type "MyType" do |t|
+              t.field "id", "ID!"
+              t.field "nested_fields_gql", "NestedFields", name_in_index: "nested_fields_index"
+              t.index "widgets"
+            end
+          end.runtime_metadata
+
+          widgets = runtime_metadata.index_definitions_by_name.fetch("widgets")
+          expect(widgets.field_paths_protected_from_removal).to eq ["nested_fields_index.old_nested_field"].to_set
+        end
+
+        it "includes declarations from all subtypes of an indexed union type" do
+          runtime_metadata = define_schema do |s|
+            s.object_type "Widget" do |t|
+              t.field "id", "ID!"
+              t.deleted_field "old_widget_field"
+            end
+
+            s.object_type "Component" do |t|
+              t.field "id", "ID!"
+              t.deleted_field "old_component_field"
+            end
+
+            s.union_type "Thing" do |t|
+              t.subtypes "Widget", "Component"
+              t.index "things"
+            end
+          end.runtime_metadata
+
+          things = runtime_metadata.index_definitions_by_name.fetch("things")
+          expect(things.field_paths_protected_from_removal).to eq ["old_widget_field", "old_component_field"].to_set
+        end
+      end
+
       def index_definition_metadata_for(name, type_definition_order: ["NestedFields", "MyType"], on_my_type: nil, **options, &block)
         type_defs = {
           "NestedFields" => ->(t) do
diff --git a/spec_support/lib/elastic_graph/spec_support/runtime_metadata_support.rb b/spec_support/lib/elastic_graph/spec_support/runtime_metadata_support.rb
index 33aee9181..3192d3321 100644
--- a/spec_support/lib/elastic_graph/spec_support/runtime_metadata_support.rb
+++ b/spec_support/lib/elastic_graph/spec_support/runtime_metadata_support.rb
@@ -117,13 +117,14 @@ def static_param_with(value)
           StaticParam.new(value: value)
         end
 
-        def index_definition_with(route_with: nil, rollover: nil, default_sort_fields: [], current_sources: [SELF_RELATIONSHIP_NAME], fields_by_path: {}, has_had_multiple_sources: false, sourced_from_nested_paths_by_qualified_relationship: {})
+        def index_definition_with(route_with: nil, rollover: nil, default_sort_fields: [], current_sources: [SELF_RELATIONSHIP_NAME], fields_by_path: {}, field_paths_protected_from_removal: [], has_had_multiple_sources: false, sourced_from_nested_paths_by_qualified_relationship: {})
           IndexDefinition.new(
             route_with: route_with,
             rollover: rollover,
             default_sort_fields: default_sort_fields,
             current_sources: current_sources,
             fields_by_path: fields_by_path,
+            field_paths_protected_from_removal: field_paths_protected_from_removal,
             has_had_multiple_sources: has_had_multiple_sources,
             sourced_from_nested_paths_by_qualified_relationship: sourced_from_nested_paths_by_qualified_relationship
           )