Merge pull request #2077 from matrix-org/rav/proposal/room_v5

MSC2077: room v5

proposals/2077-rooms-v5.md

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+# MSC2077 - Room version 5
+
+This MSC proposes creating room version 5, which will enforce the signing key
+`valid_until_ts` timestamps proposed in
+[MSC2076](https://github.com/matrix-org/matrix-doc/issues/2076).
+
+## Proposal
+
+The new room version is called `5`. The only difference between v5 and v4 is
+that v5 rooms enforce the `valid_until_ts` timestamp on signing keys as
+proposed in [MSC2076](https://github.com/matrix-org/matrix-doc/issues/2076).
+
+It is not yet proposed to change the default room version to v5. Version 5 will
+be considered a "stable" version.
+
+## Notes
+
+See also [MSC2002](./2002-rooms-v4.md), which proposed room v4 but also
+mentioned that a v5 was anticipated and gave some context for this change.