MSC4304: Room Version 12 (#4304)

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* MSC: Room Version 12

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Encourage servers to deviate from the SHOULD in the spec

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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

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Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

proposals/4304-room-version-12.md

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+# MSC4304: Room Version 12
+
+A new room version, `12`, is proposed using [room version 11](https://spec.matrix.org/v1.15/rooms/v11/)
+as a base and incorporating the following MSCs:
+
+* [MSC4289](https://github.com/matrix-org/matrix-spec-proposals/pull/4289) - Explicitly privilege room creators
+* [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291) - Room IDs as hashes of the create event
+* [MSC4297](https://github.com/matrix-org/matrix-spec-proposals/pull/4297) - State Resolution v2.1
+
+Though not technically required, [MSC4307: Validate `auth_events` are in the correct room](https://github.com/matrix-org/matrix-spec-proposals/pull/4307)
+is explicitly included in this room version as well.
+
+Other MSCs are capable of being included in this version, but they do not have sufficient implementation, acceptance,
+and/or testing to be considered stable enough for v12 rooms. A future room version may still include them. Most
+notable are:
+
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2870 - Lacking testing.
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2244 - Lacking implementation.
+
+Room version 12 upon being added to the specification shall be considered stable. No other room versions
+are affected by this MSC.
+
+Typically, an MSC like this one which cuts a room version release would not be encouraged to make that same
+room version the default for new rooms. Given the security context around the above MSCs however, this MSC
+***does*** update the default room version to be v12 immediately. A wide variety of server implementations
+already exist at the time of publishing this MSC, and major clients have been tested for compatibility with
+the room version, though there are some noted incompatibilities expected.
+
+As a result of those incompatibilities, servers are encouraged to exercise the "SHOULD" on the default room
+version applied by the spec and deviate in the early days/weeks of v12's rollout. Once their respective
+communities are better prepared, which may be a matter of days after publishing, servers SHOULD return back
+to the default in the spec.
+
+## Unstable prefix
+
+Implementations looking to test v12 before written into the specification should use `org.matrix.hydra.11`
+as the room version, treating it as unstable.
+
+## Prior art
+
+Room version MSCs are meant to be lightweight and fit a standard process. In backwards chronological
+order, they are:
+
+* https://github.com/matrix-org/matrix-spec-proposals/pull/4239 - Room version 11 (made default)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3820 - Room version 11 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3904 - Room version 10 (made default)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3604 - Room version 10 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3589 - Room version 9 (made default)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3375 - Room version 9 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/3289 - Room version 8 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2998 - Room version 7 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2788 - Room version 6 (made default)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2240 - Room version 6 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2077 - Room version 5 (creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/2002 - Room version 4 (creation; made default [retroactively](https://github.com/matrix-org/matrix-doc/pull/2082))
+* https://github.com/matrix-org/matrix-spec-proposals/pull/1943 - Room version 3 (made default; closed)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/1659 - Room version 3 (no-longer-standard inline creation)
+* https://github.com/matrix-org/matrix-spec-proposals/pull/1759 - Room version 2 (creation)
+
+Room version 1 was the first room version, released formally in https://github.com/matrix-org/matrix-doc/pull/1773 (~r0.5.0, "Matrix 1.0", June 2019)