Suggestion: end world readability of tombstoned/upgraded rooms

Nobody has claimed this yet.

**Suggestion**

There are clients that leave old versions of rooms when user follows tombstone (e.g. [FluffyChat#880](https://gitlab.com/famedly/fluffychat/-/issues/880) & https://github.com/vector-im/element-web/issues/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 https://github.com/matrix-org/matrix-public-archive/issues/47#issuecomment-1217020589 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.

Start by reviewing the Matrix specification rules for room tombstones, upgrades, history visibility, and join rules referenced by the issue. Done means the specification has a decided, implementable way to prevent inaccessible old-room history from remaining publicly readable, with the related joining behavior resolved or explicitly scoped.

Written by the indexing model from the issue text.