MSC3375: Room version 9. (#3375)

* Room version 9.

* Fix text which was swapped.

* Update stability.

proposals/3375-room-v9.md

@@ -0,0 +1,26 @@
+# MSC3375: Room Version 9
+
+A new room version, `9`, is proposed using [room version 8](https://spec.matrix.org/unstable/rooms/v8/)
+as a base and incorporating the following:
+
+The redaction rules for room version 9 are updated, such that `m.room.member`
+events allow the `join_authorised_via_users_server` key in addition to the
+`membership` key.
+
+Though other MSCs are capable of being included in this version, they do not have
+sufficient implementation to be considered for this room version. A future room
+version may include them.
+
+Room version `9` upon being added to the specification shall be considered stable.
+No other room versions are affected by this MSC.
+
+## Background
+
+Protecting this key from redaction was missing in [MSC3083](https://github.com/matrix-org/matrix-doc/blob/main/proposals/3083-restricted-rooms.md).
+By redacting it, new servers which join a room are unable to properly authorise
+`m.room.member` events which include the `join_authorised_via_users_server` key
+and have been redacted. This can cause a split-brained room where some servers
+believe a member is joined and other servers do not see the member in the room.
+
+Note that a new room version is necessary since this will change that event ID
+calculation of the `m.room.member` event (and thus is not backwards compatible).