Option to disallow unencrypted messages/events in a room

Nobody has claimed this yet.

The option to disallow unencrypted messages in a room could help ease some discomfort when clients that don't support e2e join a room, potentially leaking history. This is less of a concern for private rooms, however having plain text messages in the log of an encrypted room are a bit worrying at a glance.

Having the option is also nice for if an unsafe bot joins the room and responds in an unencrypted fashion instead of encrypting its reply. This can be combated by other procedures, however this is still a possible scenario where the option could be used.

This would be a nice-to-have feature, ideally supported as a toggle on clients. The toggle would allow for periods of presumed higher security with more relaxed valleys at other times.

The issue proposes a client-controlled option for rejecting unencrypted messages or events in encrypted rooms, but it names no files, tests, or protocol entry points. First determine where room message acceptance and encryption requirements are specified, then define the protocol and client behavior needed for the toggle and its security boundaries.

Written by the indexing model from the issue text.