Suggestion: end world readability of tombstoned/upgraded rooms

[Mikaela]

Suggestion

There are clients that leave old versions of rooms when user follows tombstone (e.g. FluffyChat#880 & element-hq/element-web#10380 (:white_check_mark:)) and history visibility isn't connected to access to the room. Thus it's possible to have public history in rooms that no one in power can join anymore and cause privacy leak, especially with projects such as matrix-static or matrix-public-archive that to my knowledge don't care about tombstones.

Originally raised at matrix-org/matrix-viewer#47 (comment) by me.

I think there was also a related suggestion somewhere to make the old room allow joining by being a member of the new room automatically (adding the new room to upgraded room's restricted or knock_restricted join rule or doing that automatically), but I am unable to locate it from this repository and it may also have been only suggested in chat and not raised at a issue tracker.

[Mikaela]

Related Spec Proposals (while trying to find the issues from previous paragraph):

• MSC2214: Joining upgraded private rooms matrix-spec-proposals#2214 I am having difficulties parsing does it affect joining the pre- or post-upgrade room.
• MSC3325: Upgrading invite-only rooms matrix-spec-proposals#3325 appears to be an answer to the reverse issue.