[vertex-four]

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.

[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.