- Add new endpoint `/_matrix/client/r0/rooms/{roomId}/aliases` from Client-Server API r0.6.1 (previously [MSC2432](https://github.com/matrix-org/matrix-doc/pull/2432)). ([\#9224](https://github.com/matrix-org/synapse/issues/9224))
- Improve performance of incoming federation transactions in large rooms. ([\#9953](https://github.com/matrix-org/synapse/issues/9953), [\#9973](https://github.com/matrix-org/synapse/issues/9973))
- Rewrite logic around verifying JSON object and fetching server keys to be more performant and use less memory. ([\#10035](https://github.com/matrix-org/synapse/issues/10035))
- Add an admin API for unprotecting local media from quarantine. Contributed by @dklimpel. ([\#10040](https://github.com/matrix-org/synapse/issues/10040))
- Add new admin APIs for unprotecting local media from quarantine. Contributed by @dklimpel. ([\#10040](https://github.com/matrix-org/synapse/issues/10040))
- Add new admin APIs to remove media by media ID from quarantine. Contributed by @dklimpel. ([\#10044](https://github.com/matrix-org/synapse/issues/10044))
- Make reason and score parameters optional for reporting content. Implements [MSC2414](https://github.com/matrix-org/matrix-doc/pull/2414). Contributed by Callum Brown. ([\#10077](https://github.com/matrix-org/synapse/issues/10077))
- Add support for routing more requests to workers. ([\#10084](https://github.com/matrix-org/synapse/issues/10084))
