feat!: delete richtext-slate (#16359)

**BREAKING:** This PR removes richtext-slate - starting in Payload 4.0,
richtext-lexical is our only supported rich text editor. If you're still
using richtext-slate, please migrate to lexical **before** upgrading to
4.0. If you're not using richtext-slate, this change will not affect you

Slate has been deprecated for a while and we haven't been investing in
it. Removing it cuts a lot of legacy code, reduces the maintenance
overhead and lets us focus rich text work on a single editor going
forward.


## What's removed

- **`@payloadcms/richtext-slate`** package - deleted entirely. It will
no longer be published.
- **Slate → Lexical migration utilities** inside
`@payloadcms/richtext-lexical/migrate`
- **`payload-plugin-lexical` migration utilities**
- **Docs** — all Slate references pruned from the documentation

## What stays

- The **rich text adapter pattern** is unchanged - `editor:
lexicalEditor({})` continues to work exactly as before, and third-party
adapters following the same interface continue to be supported.

## Migration path for users

If you're still on Slate, **do not upgrade to 4.0 yet.** Stay on the
latest 3.x release and follow the [slate => lexical migration
guide](https://payloadcms.com/docs/rich-text/migration).

---
- To see the specific tasks where the Asana app for GitHub is being
used, see below:
  - https://app.asana.com/0/0/1214207842694534

---------

Co-authored-by: Copilot <copilot@github.com>

Author: AlessioGr

.github/ISSUE_TEMPLATE/1.bug_report_v3.yml

@@ -56,7 +56,6 @@ body:
         - 'plugin: nested-docs'
         - 'plugin: redirects'
         - 'plugin: richtext-lexical'
-        - 'plugin: richtext-slate'
         - 'plugin: search'
         - 'plugin: sentry'
         - 'plugin: seo'

.github/workflows/pr-title.yml

@@ -71,7 +71,6 @@ jobs:
             plugin-stripe
             richtext-\*
             richtext-lexical
-            richtext-slate
             sdk
             storage-\*
             storage-azure

.idea/payload.iml

@@ -15,7 +15,7 @@ Payload is a monorepo structured around Next.js, containing the core CMS platfor
   - `packages/db-*` - Database adapters (MongoDB, Postgres, SQLite, Vercel Postgres, D1 SQLite)
   - `packages/drizzle` - Drizzle ORM integration
   - `packages/kv-redis` - Redis key-value store adapter
-  - `packages/richtext-*` - Rich text editors (Lexical, Slate)
+  - `packages/richtext-*` - Rich text editors (Lexical)
   - `packages/storage-*` - Storage adapters (S3, Azure, GCS, Uploadthing, Vercel Blob, R2)
   - `packages/email-*` - Email adapters (Nodemailer, Resend)
   - `packages/plugin-*` - Additional functionality plugins

CLAUDE.md

@@ -162,9 +162,9 @@ This is the UI library that Payload's Admin Panel uses. All components are expor
 
 You can choose which Database Adapter you'd like to use for your project, and no matter which you choose, the entire data layer for Payload is contained within these packages. You can only use one at a time for any given project.
 
-`@payloadcms/richtext-lexical`, `@payloadcms/richtext-slate`
+`@payloadcms/richtext-lexical`
 
-Payload's Rich Text functionality is abstracted into separate packages and if you want to enable Rich Text in your project, you'll need to install one of these packages. We recommend Lexical for all new projects, and this is where Payload will focus its efforts on from this point, but Slate is still supported if you have already built with it.
+Payload's Rich Text functionality is abstracted into a separate package. If you want to enable Rich Text in your project, you'll need to install this package.
 
 <Banner type="info">
   **Note:** Rich Text is entirely optional and you may not need it for your

docs/getting-started/concepts.mdx

@@ -52,7 +52,6 @@ For more details, see the [Documentation](https://payloadcms.com/docs/getting-st
 
    - @payloadcms/db-mongodb
    - @payloadcms/db-postgres
-   - @payloadcms/richtext-slate
    - @payloadcms/richtext-lexical
    - @payloadcms/plugin-form-builder
    - @payloadcms/plugin-nested-docs

docs/migration-guide/overview.mdx

@@ -6,21 +6,11 @@ order: 10
 title: Rich Text Editor
 ---
 
-<Banner type="success">
+Payload's rich text editor powers the [rich text field](/docs/fields/rich-text). It is built on top of [Lexical](https://lexical.dev/), Meta's rich text framework, which provides the underlying foundation we extend with Payload-specific features.
 
-This documentation is about our new editor, based on Lexical (Meta's rich text editor). The previous default
-editor, based on Slate, has been deprecated and will be removed in 4.0. You can read [its documentation](/docs/rich-text/slate),
-or the [migration guide](/docs/rich-text/migration) to migrate from Slate to Lexical (recommended).
+We aim to offer the best editing experience you can imagine: healthy defaults out of the box, paired with the flexibility to customize every detail — from the “/” menu and toolbars (inline or fixed) to inserting any component or subfield you need.
 
-</Banner>
-
-The editor is the most important property of the [rich text field](/docs/fields/rich-text).
-
-As a key part of Payload, we are proud to offer you the best editing experience you can imagine. With healthy
-defaults out of the box, but also with the flexibility to customize every detail: from the “/” menu
-and toolbars (whether inline or fixed) to inserting any component or subfield you can imagine.
-
-To use the rich text editor, first you need to install it:
+To use the rich text editor, first install it:
 
 ```bash
 pnpm install @payloadcms/richtext-lexical