docs: add ipfs pubsub deprecation reminder to changelog (#9827)

docs/changelogs/v0.20.md

@@ -13,6 +13,7 @@
     - [Trace Context support](#trace-context-support)
     - [Removed legacy features](#removed-legacy-features)
   - [`--empty-repo` is now the default](#--empty-repo-is-now-the-default)
+  - [Reminder: `ipfs pubsub` commands and matching HTTP endpoints are deprecated and will be removed](#reminder-ipfs-pubsub-commands-and-matching-http-endpoints-are-deprecated-and-will-be-removed)
 - [📝 Changelog](#-changelog)
 - [👨‍👩‍👧‍👦 Contributors](#-contributors)
 
@@ -109,6 +110,10 @@ When creating a repository with `ipfs init`, `--empty-repo=true` is now the defa
 that your repository will be empty by default instead of containing the introduction files.
 You can read more about the rationale behind this decision on the [tracking issue](https://github.com/ipfs/kubo/issues/9757).
 
+#### Reminder: `ipfs pubsub` commands and matching HTTP endpoints are deprecated and will be removed
+
+`ipfs pubsub` commands and all `/api/v0/pubsub/` RPC endpoints and will be removed in the next release. For more information and rational see [#9717](https://github.com/ipfs/kubo/issues/9717).
+
 ### 📝 Changelog
 
 ### 👨‍👩‍👧‍👦 Contributors