Update redaction rules.

Author: clokep

proposals/3289-rooms-v8.md

@@ -13,3 +13,10 @@ version may include them.
 Room version `8` upon being added to the specification shall be considered stable.
 No other room versions are affected by this MSC. Before room version `8` can enter
 the specification, MSC3083 needs to complete its final comment period.
+
+## Updated redaction rules
+
+Room version 8 shall update the redaction rules for the `m.room.join_rules` event.
+[Room version 1](https://spec.matrix.org/unstable/rooms/v1/#redactions) defines
+that only the `join_rule` key in the content object should be kept. Room version
+8 defines that both the `join_rule` and `allow` keys of the content shall be kept.