[x0x0]

Not cool.

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.

[vertex-four]

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.