NIP-104 is MLS (a newish messaging protocol) over Nostr. nostr:npub1zuuajd7u3sx8xu92yav9jwxpr839cs0kc3q6t56vd5u9q033xmhsk6c2uc speaks about it here: https://youtu.be/PWKd9aoZ-Cg

Looks very promising, but not ready to be implemented into clients yet.