there aren't downgrade attacks. we turned on E2EE by default in May for private rooms, and there's no negotiation involved. if you're on a client that supports E2EE (i.e. almost all major ones, now) and you try to DM someone, they simply won't be able to read you unless they support E2EE. i.e. they can't downgrade the convo.

That's good. The last time I had a look at Matrix clients it was a mess. IIUC the E2EE isn't enabled by default for the old Riot client, only RiotX and Riot web have it.

What happens if someone with old Riot client creates a room and someone with e.g. RiotX joins it, will it force E2EE on? Or will it fall back to non-E2EE messaging?

The creator and admins of the room picks the encryption preferences, iiuc: if you have a client that doesn’t support E2EE, you might be able to create an encrypted room (?) but it would be pretty useless. The clients all clearly mark the encryption status of the room you’re in.

So if an ignorant/malicious user creates a room without E2EE and doesn't care to enable it even when requested, all users are forced to converse in effectively plaintext, and the solution is "clients tell users it's not E2EE".

IMO it should be the case that it's always E2EE, no other options. Until that's the case I think Matrix ecosystem isn't keeping up with centralized solutions like Signal.

E2EE is really annoying, in lots of ways: if the users in the room want encryption, I’d rather they just create a new room.

Yeah, I really like the ability to have an unencrypted channel: easy bots and bridges are one of the main advantages for me of matrix vs. IRC.

My only big issue is that the iOS client doesn’t support multiple simultaneous identities.

I should have said “easy bots and bridges are one of the main advantages for me of matrix as a successor to IRC”