Support Matrix 1.16

Following on from https://github.com/element-hq/synapse/issues/18731.

# Matrix Specification v1.16 Changelog

| | |
|---|---|
| Git commit | https://github.com/matrix-org/matrix-spec/tree/v1.16 |
| Release date | September 17, 2025 |

## Client-Server API

**Deprecations**

- [x] Deprecate `m.set_avatar_url` and `m.set_displayname` capabilities, as per [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133). ([#2071](https://github.com/matrix-org/matrix-spec/issues/2071))

**Removed Endpoints**

- [x] Remove unintentional intentional mentions in replies, as per [MSC4142](https://github.com/matrix-org/matrix-spec-proposals/pull/4142). ([#2210](https://github.com/matrix-org/matrix-spec/issues/2210))

**Backwards Compatible Changes**

- [x] Update user profile endpoints to handle custom fields, and add a new `m.profile_fields` capability, as per [MSC4133](https://github.com/matrix-org/matrix-spec-proposals/pull/4133). ([#2071](https://github.com/matrix-org/matrix-spec/issues/2071))
- [x] Add `format` query parameter to `GET /state/{eventType}/{stateKey}` to allow fetching metadata of a specific state event. ([#2175](https://github.com/matrix-org/matrix-spec/issues/2175))
- [ ] Add the `use_state_after` query parameter and `state_after` response property to `GET /sync`, as per [MSC4222](https://github.com/matrix-org/matrix-spec-proposals/issues/4222). ([#2187](https://github.com/matrix-org/matrix-spec/issues/2187))
- [x] When upgrading rooms to [room version 12](/rooms/v12), `additional_creators` may be specified on the [`POST /_matrix/client/v3/rooms/{roomId}/upgrade`](/client-server-api/#post_matrixclientv3roomsroomidupgrade) endpoint, as per [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] When creating rooms with [room version 12](/rooms/v12), the `trusted_private_chat` preset should merge the invitees into the supplied `content.additional_creators` in the resulting room, as per [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] In [room version 12](/rooms/v12), the power level of room creators is now infinitely high as per [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] In [room version 12](/rooms/v12), room IDs no longer have a domain component as per [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] When creating rooms with [room version 12](/rooms/v12), the initial power levels will restrict the ability to upgrade rooms by default, as per [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [ ] In [room version 12](/rooms/v12), room creators should be to re-join a room where everyone has left, https://github.com/matrix-org/complement/pull/843 (Edit: this isn't required and is non-obvious how to do it properly / with a decent UX)
- [x] Add a profile field for a user's time zone, as per [MSC4175](https://github.com/matrix-org/matrix-spec-proposals/pull/4175). ([#2206](https://github.com/matrix-org/matrix-spec/issues/2206))
- [x] Invites and knocks are now expected to contain the `m.room.create` event in their stripped state entries, as per [MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311). ([#2207](https://github.com/matrix-org/matrix-spec/issues/2207)) -> https://github.com/element-hq/synapse/pull/19722

**Spec Clarifications**

- [x] Clarify that `format` is required if `formatted_body` is specified. ([#2167](https://github.com/matrix-org/matrix-spec/issues/2167))
- [x] The `latest_event` in an aggregated set of thread events uses the same format as the event itself. ([#2169](https://github.com/matrix-org/matrix-spec/issues/2169))
- [x] Fix various typos throughout the specification. ([#2171](https://github.com/matrix-org/matrix-spec/issues/2171), [#2177](https://github.com/matrix-org/matrix-spec/issues/2177), [#2179](https://github.com/matrix-org/matrix-spec/issues/2179))
- [x] Clarify that clients should replace events with the most recent replacement by `origin_server_ts`. ([#2190](https://github.com/matrix-org/matrix-spec/issues/2190))
- [x] Fix `/sync` flow referencing incorrect parameter to use with `/messages`. ([#2195](https://github.com/matrix-org/matrix-spec/issues/2195))
- [x] Clarify wording around the `world_readable` history visibility setting. Contributed by @HarHarLinks. ([#2204](https://github.com/matrix-org/matrix-spec/issues/2204))

## Server-Server API

**Backwards Compatible Changes**

- [ ] `invite_room_state` and `knock_room_state` now have additional requirements and validation depending on the room version, as per [MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311) (including the full PDU event format). ([#2207](https://github.com/matrix-org/matrix-spec/issues/2207)) -> https://github.com/element-hq/synapse/pull/19723

## Application Service API

No significant changes.

## Identity Service API

No significant changes.

## Push Gateway API

No significant changes.

## Room Versions

**Backwards Compatible Changes**

- [x] Room IDs in room version 12 are now the event ID of the create event with the normal room ID sigil (`!`), as per [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] Room creators are formalized in room version 12 and have infinitely high power level, as per [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] State Resolution is updated in room version 12 to reduce the opportunity for "state resets", as per [MSC4297](https://github.com/matrix-org/matrix-spec-proposals/pull/4297). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [ ] The default room version is now room version 12, though servers SHOULD keep using room version 11 for a little while, as per [MSC4304](https://github.com/matrix-org/matrix-spec-proposals/pull/4304). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))
- [x] Add [room version 12](/rooms/v12) as per [MSC4304](https://github.com/matrix-org/matrix-spec-proposals/pull/4304). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193), [#2199](https://github.com/matrix-org/matrix-spec/issues/2199))

**Spec Clarifications**

- [x] In room versions 1 through 12, an event's `auth_events` have always needed to belong to the same room as per [MSC4307](https://github.com/matrix-org/matrix-spec-proposals/pull/4307). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))

## Appendices

**Backwards Compatible Changes**

- [x] Room IDs can now appear without a domain component in [room version 12](/rooms/v12), as per [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291). ([#2193](https://github.com/matrix-org/matrix-spec/issues/2193))

## Internal Changes/Tooling

**Backwards Compatible Changes**

- [x] Add "placeholder MSC" process definition. ([#2157](https://github.com/matrix-org/matrix-spec/issues/2157))

**Spec Clarifications**

- [x] Declare the Application Service Registration schema to follow JSON Schema spec 2020-12. ([#2132](https://github.com/matrix-org/matrix-spec/issues/2132))
- [x] Declare the event schemas to follow JSON Schema spec 2020-12. ([#2132](https://github.com/matrix-org/matrix-spec/issues/2132))
- [x] Upgrade the docsy theme to version 0.12.0. ([#2160](https://github.com/matrix-org/matrix-spec/issues/2160))
- [x] GitHub actions are now building the OpenAPI `spec/identity-service-api/api.json` file. ([#2172](https://github.com/matrix-org/matrix-spec/issues/2172))
- [x] Minor fixes to JSON schemas. ([#2182](https://github.com/matrix-org/matrix-spec/issues/2182))
- [x] Specify a correct spelling for "display name". ([#2189](https://github.com/matrix-org/matrix-spec/issues/2189))
- [x] Fix a grammatical typo on the Matrix Spec Process documentation page. ([#2205](https://github.com/matrix-org/matrix-spec/issues/2205))

Start by reviewing the unchecked Matrix 1.16 requirements for GET /sync, server-server invite_room_state and knock_room_state validation, and the default room version. Check the linked Synapse pull requests in the issue before starting; done means the remaining requirements are implemented and covered by the relevant tests.

Written by the indexing model from the issue text.