I would also challenge OP on the relevance of E2EE for very large MUCs: beyond a certain point, the room can't reasonably be considered private. With current (S

I would also challenge OP on the relevance of E2EE for very large MUCs: beyond a certain point, the room can't reasonably be considered private. With current (Signal-style, also found in XMPP, Matrix, …) crypto, you are encoding ever message for every user/client joined, which scales as poorly as one can guess, but is still adequate for small groupchats. MLS stands to rise that threshold (and there's work in progress to include it in XMPP), but that certainly won't address the fact that this is just privacy theater.