White Noise, which is based on the MLS protocol, uses Nostr to transmit the data. It's entirely decentralised. It has good support for large groups This podcast episode is a nice intro:

Fountain

Citadel Dispatch • CD166: JEFF GARDNER AND MAX HILLEBRAND - WHITE NOISE PRIVATE CHAT • Listen on Fountain

White Noise is a secure private messenger that aims to provide similar functionality to Signal in a more open, interoperable, and robust way. The a...

I'm very impressed. But it's still relatively new. The MLS protocol is not new, it seems to be well regarded. But the rolling out into the real world is still in the early stages