This is remarkably contrary to how people actually use software. What people see is "click this button to make annoying red flashing shit stop so I can do what I want to do".
There's a reason web browsers just don't allow users to easily get past the annoying pages when there is a chance they're being attacked. I see no reason that Matrix clients would be required to allow users to break security without having a persistent banner saying "this room is insecure".
Cool. If they ignore the great big banner which says "do not enter any personal info, bank info, etc etc into this window" and they're attacked, obviously they didn't care much. In the meantime, people who actually understand security can make a reasonable decision.
First, that's why people like Signal: it just works (TM) encryption with no user gotchas.
Second, any communication is only as encrypted / safe as the minimum of the people with access to it. So if someone ignores warnings and enters that chatroom, he or she puts everyone at risk. Because sometimes she/he really is being MITM or surveilled by someone/oppressive government du jour.
The point is that you wouldn't be able to enter a chatroom at a higher version than your server+client supports - how would the old code be able to understand it, after all? You'd be in pre-upgrade chatrooms, which would display the banner for everyone until relevant people upgrade/get kicked, and you could possibly start new chats with people, which would display the banner for all participants, but if you were on version 5 and #megolm:matrix.org was on version 6, you just couldn't join it until you upgraded.
