MSC3821: Update the redaction rules, again

proposals/3821-update-redaction-rules-again.md

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+# Update redaction rules, again
+
+The current [redaction algorithm](https://spec.matrix.org/v1.6/rooms/v9/#redactions) are
+still missing some key fields, even after [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176)
+attempted to fix some of the obvious cases. This MSC aims to fix more fields.
+
+## Proposal
+
+*Note*: It is recommended to read [MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176)
+before this proposal as MSC2176 contains a lot of backing context.
+
+In a new room version due to redactions affecting event hashes, we:
+
+* `m.room.member` events additionally preserve part of `third_party_invite` under `content`, if present:
+  * Spec: https://spec.matrix.org/v1.6/client-server-api/#mroommember
+  * Under `third_party_invite`, `signed` is preserved. Rationale being that the `signed` block is required
+    for the server to validate the event, however excess fields and `display_name` are not as important.
+    Clients which are trying to represent a redacted `m.room.member` event with `third_party_invite` should
+    not rely on `display_name` being present but rather state "Bob accepted a third party invite" or similar.
+
+## Unstable prefix
+
+Implementations looking to test these changes before adopted into a stable room version should use
+`org.matrix.msc3821.opt1` as the room version, using v9 as a base and treating it as unstable.