MSC3604: Room Version 10 (#3604)

* WIP: Template out v10

* Add opt1 room version prefix

* Update 3604-rooms-v10.md

* Upgrade MSC3667

* Cut a v10

* Move MSC2176 to "not feasible" section

proposals/3604-rooms-v10.md

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+# MSC3604: Room Version 10
+
+A new room version, `10`, is proposed using [room version 9](https://spec.matrix.org/v1.2/rooms/v9/) as a base
+and incorporating the following MSCs:
+
+* [MSC3667](https://github.com/matrix-org/matrix-spec-proposals/pull/3667) - Enforcing integer parsing on power levels
+* [MSC3787](https://github.com/matrix-org/matrix-spec-proposals/pull/3787) - Allowing knocks to restricted rooms
+
+Though other MSCs are capable of being included in this version, they do not have sufficient implementation to be
+considered stable enough for v10 rooms. A future room version may still include them. Such examples of ineligible MSCs
+are:
+
+* [MSC2244](https://github.com/matrix-org/matrix-spec-proposals/pull/2244) - Mass redactions
+* [MSC2174](https://github.com/matrix-org/matrix-spec-proposals/pull/2174) - Move `redacts` to sane place
+* [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176) - Update to redaction rules
+* [MSC2175](https://github.com/matrix-org/matrix-spec-proposals/pull/2175) - Remove extraneous `creator` field
+* [MSC2828](https://github.com/matrix-org/matrix-spec-proposals/pull/2828) - Restrict allowed user IDs over federation
+* [MSC1229](https://github.com/matrix-org/matrix-spec-proposals/issues/1229) - Mitigating abuse of `depth`
+
+Room version 10 upon being added to the specification shall be considered stable. No other room versions are affected
+by this MSC.
+
+## Unstable prefix
+
+Implementations looking to test v10 before written into the specification should use `org.matrix.msc3604.opt3`
+as the room version, treating it as unstable.
+
+Other prefixes in the wild:
+* `org.matrix.msc3604.opt1`: `v9` + MSC2176 (https://github.com/matrix-org/synapse/pull/11662)
+* `org.matrix.msc3604.opt2`: `v9` + MSC2176 + MSC3667 (https://github.com/matrix-org/synapse/pull/11885)