MSC3821: Update the redaction rules, again (#3821)

* Start an MSC2176 2.0

* Remove scary warning about unstable implementation

* Update spec links

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Rewrite words into order that sense make

* Update intro & unstable prefix history

---------

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

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

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+# MSC3821: Update redaction rules, again
+
+[MSC2176](https://github.com/matrix-org/matrix-spec-proposals/pull/2176) aimed to fix inconsistencies
+with the [redaction algorithm](https://spec.matrix.org/v1.6/rooms/v10/#redactions) where server-side
+event auth required properties to exist that were being removed. While MSC2176 fixed a number of cases,
+one was unfortunately missed.
+
+This MSC aims to fix that missing case, originally identified long ago by [synapse#1831](https://github.com/matrix-org/synapse/issues/1831).
+
+## 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 future room version, the following changes are made to the [redaction algorithm](https://spec.matrix.org/v1.6/rooms/v10/#redactions).
+Note that this requires a new room version because changing the redaction algorithm changes how
+[event IDs](https://spec.matrix.org/v1.6/rooms/v10/#event-ids) are calculated, as they are
+[reference hashes](https://spec.matrix.org/v1.6/server-server-api/#calculating-the-reference-hash-for-an-event)
+which redact the event during calculation.
+
+* [`m.room.member`](https://spec.matrix.org/v1.6/client-server-api/#mroommember) events preserve a portion
+  of `third_party_invite` under `content`, if present. Those properties being:
+
+  * `signed`: the block is required for the server to validate the event, however excess adjacent properties
+    such as `display_name` are not important.
+
+Clients should note that because `display_name` is *not* preserved during redaction they might need to change
+how that event is rendered/presented to users. For example, when rendering such a redacted event the client
+might show it as "Bob accepted a third party invite".
+
+## 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.
+
+**History**: This MSC was originally written before room version 10 existed. Implementations wishing to use
+v10 as a base instead of v9 should use `org.matrix.msc3821.opt1.v10` as the room version instead.