### Client-Server API
Breaking Changes
- The `prev_content` field is now returned inside the `unsigned` property of events, rather than at the top level, as per [MSC3442](https://github.com/matrix-org/matrix-doc/pull/3442). ([#3524](https://github.com/matrix-org/matrix-doc/issues/3524))
- The `aliases` property from the chunks returned by `/publicRooms`, as per [MSC2432](https://github.com/matrix-org/matrix-doc/pull/2432). ([#3624](https://github.com/matrix-org/matrix-doc/issues/3624))
New Endpoints
- Add the Space Hierarchy API (`GET /_matrix/client/v1/rooms/{roomId}/hierarchy`) as per [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946). ([#3610](https://github.com/matrix-org/matrix-doc/issues/3610))
- Add `/_matrix/client/v1/register/m.login.registration_token/validity` as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231). ([#3616](https://github.com/matrix-org/matrix-doc/issues/3616))
Backwards Compatible Changes
- Extend `/_matrix/client/r0/login` to accept a `m.login.appservice`, as per [MSC2778](https://github.com/matrix-org/matrix-doc/pull/2778). ([#3324](https://github.com/matrix-org/matrix-doc/issues/3324))
- Add support for `restricted` rooms as per [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083), [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289), and [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))
- Add `is_guest` to `/account/whoami` as per [MSC3069](https://github.com/matrix-org/matrix-doc/pull/3069). ([#3605](https://github.com/matrix-org/matrix-doc/issues/3605))
- Expand guest access to sending any room event and state event as per [MSC3419](https://github.com/matrix-org/matrix-doc/pull/3419). ([#3605](https://github.com/matrix-org/matrix-doc/issues/3605))
- Add Spaces and room types as per [MSC1772](https://github.com/matrix-org/matrix-doc/pull/1772) and [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946). ([#3610](https://github.com/matrix-org/matrix-doc/issues/3610))
- Add new `m.set_displayname`, `m.set_avatar_url`, and `m.3pid_changes` capabilities as per [MSC3283](https://github.com/matrix-org/matrix-doc/pull/3283). ([#3614](https://github.com/matrix-org/matrix-doc/issues/3614))
- Add support for fallback keys (optional keys used once one-time keys run out), as per [MSC2732](https://github.com/matrix-org/matrix-doc/pull/2732). ([#3615](https://github.com/matrix-org/matrix-doc/issues/3615))
- Add token-authenticated registration support as per [MSC3231](https://github.com/matrix-org/matrix-doc/pull/3231). ([#3616](https://github.com/matrix-org/matrix-doc/issues/3616))
Spec Clarifications
- Make `AesHmacSha2KeyDescription` consistent with `KeyDescription` in marking `name` as optional. ([#3481](https://github.com/matrix-org/matrix-doc/issues/3481))
- Fix various typos throughout the specification. ([#3482](https://github.com/matrix-org/matrix-doc/issues/3482), [#3495](https://github.com/matrix-org/matrix-doc/issues/3495), [#3509](https://github.com/matrix-org/matrix-doc/issues/3509), [#3535](https://github.com/matrix-org/matrix-doc/issues/3535), [#3591](https://github.com/matrix-org/matrix-doc/issues/3591), [#3601](https://github.com/matrix-org/matrix-doc/issues/3601), [#3611](https://github.com/matrix-org/matrix-doc/issues/3611), [#3671](https://github.com/matrix-org/matrix-doc/issues/3671), [#3680](https://github.com/matrix-org/matrix-doc/issues/3680))
- Explicitly mention RFC5870 in the definition of `m.location` events ([#3492](https://github.com/matrix-org/matrix-doc/issues/3492))
- Add `403 M_FORBIDDEN` error code to `/profile/{userId}` as per [MSC3550](https://github.com/matrix-org/matrix-doc/pull/3550). ([#3530](https://github.com/matrix-org/matrix-doc/issues/3530))
- Clarify the description of the `/sync` API, including a fix to an ASCII art diagram. ([#3543](https://github.com/matrix-org/matrix-doc/issues/3543))
- Clarify that `base_url` in client `well_known` may or may not include trailing slash. ([#3562](https://github.com/matrix-org/matrix-doc/issues/3562))
- Clarify which signature to check when decrypting `m.olm.v1.curve25519-aes-sha2` messages. ([#3573](https://github.com/matrix-org/matrix-doc/issues/3573))
- Clarify what "Stripped State" is and what purpose it serves, as per [MSC3173](https://github.com/matrix-org/matrix-doc/pull/3173). ([#3606](https://github.com/matrix-org/matrix-doc/issues/3606))
- Clarify how to interpret missing one-time key counts. ([#3636](https://github.com/matrix-org/matrix-doc/issues/3636))
- Correct the schema for the responses for various API endpoints. ([#3650](https://github.com/matrix-org/matrix-doc/issues/3650))
- Clarify that group mentions are no longer in the specification. ([#3652](https://github.com/matrix-org/matrix-doc/issues/3652))
- Distinguish between "federation" event format as exchanged by the Federation API, and the "client" event formats as used in the client-server and AS APIs. ([#3658](https://github.com/matrix-org/matrix-doc/issues/3658))
- Fix the rendering of the responses for various API endpoints. ([#3674](https://github.com/matrix-org/matrix-doc/issues/3674))
### Server-Server API
New Endpoints
- Add the Space Hierarchy API (`GET /_matrix/federation/v1/hierarchy/{roomId}`) as per [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946). ([#3610](https://github.com/matrix-org/matrix-doc/issues/3610), [#3660](https://github.com/matrix-org/matrix-doc/issues/3660))
Backwards Compatible Changes
- Add support for `restricted` rooms as per [MSC3083](https://github.com/matrix-org/matrix-doc/pull/3083), [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289), and [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))
Spec Clarifications
- Fix various typos throughout the specification. ([#3527](https://github.com/matrix-org/matrix-doc/issues/3527))
- Clarify that `GET /_matrix/federation/v1/event_auth/{roomId}/{eventId}` does *not* return the auth chain for the full state of the room. ([#3583](https://github.com/matrix-org/matrix-doc/issues/3583))
- Fix the rendering of the responses for various API endpoints. ([#3674](https://github.com/matrix-org/matrix-doc/issues/3674))
### Application Service API
Spec Clarifications
- Distinguish between "federation" event format as exchanged by the Federation API, and the "client" event formats as used in the client-server and AS APIs. ([#3658](https://github.com/matrix-org/matrix-doc/issues/3658))
- Fix the rendering of the responses for various API endpoints. ([#3674](https://github.com/matrix-org/matrix-doc/issues/3674))
- Correct the documentation for the response value for `GET /_matrix/app/v1/thirdparty/protocol/{protocol}`. ([#3675](https://github.com/matrix-org/matrix-doc/issues/3675))
### Identity Service API
Backwards Compatible Changes
- Add the `room_type` to stored invites as per [MSC3288](https://github.com/matrix-org/matrix-doc/pull/3288). ([#3610](https://github.com/matrix-org/matrix-doc/issues/3610))
Spec Clarifications
- Fix the rendering of the responses for various API endpoints. ([#3674](https://github.com/matrix-org/matrix-doc/issues/3674))
### Push Gateway API
Spec Clarifications
- Fix the rendering of the responses for various API endpoints. ([#3674](https://github.com/matrix-org/matrix-doc/issues/3674))
### Room Versions
Backwards Compatible Changes
- Add Room Version 8 as per [MSC3289](https://github.com/matrix-org/matrix-doc/pull/3289). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))
- Add Room Version 9 as per [MSC3375](https://github.com/matrix-org/matrix-doc/pull/3375). ([#3387](https://github.com/matrix-org/matrix-doc/issues/3387))
Spec Clarifications
- Fully specify room versions to indicate what exactly is carried over from parent versions. ([#3432](https://github.com/matrix-org/matrix-doc/issues/3432), [#3661](https://github.com/matrix-org/matrix-doc/issues/3661))
- Clarifications to sections on event IDs and event formats. ([#3501](https://github.com/matrix-org/matrix-doc/issues/3501))
- Remove a number of fields which were incorrectly shown to form part of the `unsigned` data of a Federation PDU. ([#3522](https://github.com/matrix-org/matrix-doc/issues/3522))
- Fix heading order of room version specifications to be consistent. ([#3683](https://github.com/matrix-org/matrix-doc/issues/3683))
- Add missing "Signing key validity period" section to room version 6. ([#3683](https://github.com/matrix-org/matrix-doc/issues/3683))
- Fix auth rules to allow membership of `knock` -> `leave` in v7, v8, and v9. ([#3694](https://github.com/matrix-org/matrix-doc/issues/3694))
### Appendices
Backwards Compatible Changes
- Describe "Common Namespaced Identifier Grammar" as per [MSC2758](https://github.com/matrix-org/matrix-doc/pull/2758). ([#3171](https://github.com/matrix-org/matrix-doc/issues/3171))
- Describe the `matrix:` URI scheme as per [MSC2312](https://github.com/matrix-org/matrix-doc/pull/2312). ([#3608](https://github.com/matrix-org/matrix-doc/issues/3608))