- Add `m.id_access_token` to `unstable_features` in `/versions` as per [MSC2264](https://github.com/matrix-org/matrix-doc/pull/2264). ([\#5974](https://github.com/matrix-org/synapse/issues/5974))
- Use the v2 Identity Service API for 3PID invites. ([\#5979](https://github.com/matrix-org/synapse/issues/5979))
- Add `POST /_matrix/client/unstable/account/3pid/unbind` endpoint from [MSC2140](https://github.com/matrix-org/matrix-doc/pull/2140) for unbinding a 3PID from an identity server without removing it from the homeserver user account. ([\#5980](https://github.com/matrix-org/synapse/issues/5980), [\#6062](https://github.com/matrix-org/synapse/issues/6062))
)
- Use `account_threepid_delegate.email` and `account_threepid_delegate.msisdn` for validating threepid sessions. ([\#6011](https://github.com/matrix-org/synapse/issues/6011))
- Allow homeserver to handle or delegate email validation when adding an email to a user's account. ([\#6042](https://github.com/matrix-org/synapse/issues/6042))
- Implement new Client Server API endpoints `/account/3pid/add` and `/account/3pid/bind` as per [MSC2290](https://github.com/matrix-org/matrix-doc/pull/2290). ([\#6043](https://github.com/matrix-org/synapse/issues/6043))
- Add an unstable feature flag for separate add/bind 3pid APIs. ([\#6044](https://github.com/matrix-org/synapse/issues/6044))
- Remove `bind` parameter from Client Server POST `/account` endpoint as per [MSC2290](https://github.com/matrix-org/matrix-doc/pull/2290/). ([\#6067](https://github.com/matrix-org/synapse/issues/6067))
- Add `POST /add_threepid/msisdn/submit_token` endpoint for proxying submitToken on an account_threepid_handler. ([\#6078](https://github.com/matrix-org/synapse/issues/6078))
- Add `POST /add_threepid/msisdn/submit_token` endpoint for proxying submitToken on an `account_threepid_handler`. ([\#6078](https://github.com/matrix-org/synapse/issues/6078))
- Add `submit_url` response parameter to `*/msisdn/requestToken` endpoints. ([\#6079](https://github.com/matrix-org/synapse/issues/6079))
- Add `m.require_identity_server` flag to /version's unstable_features. ([\#5972](https://github.com/matrix-org/synapse/issues/5972))
