docs(changelog): add redirects cleanup entry

docs/changelogs/v0.21.md

@@ -7,9 +7,10 @@
 - [Overview](#overview)
 - [🔦 Highlights](#-highlights)
   - [Saving previously seen nodes for later bootstrapping](#saving-previously-seen-nodes-for-later-bootstrapping)
-  - [`Gateway.DeserializedResponses` config flag](#gatewaydeserializedresponses-config-flag)
+  - [Gateway: `DeserializedResponses` config flag](#gateway-deserializedresponses-config-flag)
   - [`client/rpc` migration of `go-ipfs-http-client`](#clientrpc-migration-of-go-ipfs-http-client)
   - [Gateway: DAG-CBOR/-JSON previews and improved error pages](#gateway-dag-cbor-json-previews-and-improved-error-pages)
+  - [Gateway: subdomain redirects are now `text/html`](#gateway-subdomain-redirects-are-now-texthtml)
 - [📝 Changelog](#-changelog)
 - [👨‍👩‍👧‍👦 Contributors](#-contributors)
 
@@ -32,7 +33,7 @@ enabled.
 With this update, the same level of robustness is applied to peers that lack
 mDNS peers and solely rely on the public DHT.
 
-#### `Gateway.DeserializedResponses` config flag
+#### Gateway: `DeserializedResponses` config flag
 
 This release introduces the
 [`Gateway.DeserializedResponses`](https://github.com/ipfs/kubo/blob/master/docs/config.md#gatewaydeserializedresponses)
@@ -84,6 +85,23 @@ HTML responses are returned when request's `Accept` header includes `text/html`.
 | ---- | ---- |
 | ![DAG-CBOR Preview](https://github.com/ipfs/boxo/assets/5447088/973f05d1-5731-4469-9da5-d1d776891899) | ![Error Page](https://github.com/ipfs/boxo/assets/5447088/14c453df-adbc-4634-b038-133121914550) |
 
+#### Gateway: subdomain redirects are now `text/html`
+
+HTTP 301 redirects [from path to subdomain](https://specs.ipfs.tech/http-gateways/subdomain-gateway/#migrating-from-path-to-subdomain-gateway)
+no longer include the target data in the body.
+The data is returned only once, with the final HTTP 200 returned from the
+target subdomain.
+
+The HTTP 301 body now includes human-readable `text/html` message
+for clients that do not follow redirects by default:
+
+```console
+$ curl "https://subdomain-gw.example.net/ipfs/${cid}/"
+<a href="http://${cid}.ipfs.subdomain-gw.example.net/">Moved Permanently</a>.
+```
+
+Rationale can be found in [kubo#9913](https://github.com/ipfs/kubo/pull/9913).
+
 ### 📝 Changelog
 
 ### 👨‍👩‍👧‍👦 Contributors