XMPP lol. I have no idea about chat protocols, never looked into them. I hope that a chat protocol as good as Nostr comes out of all this :D
XMPP is from 1999 and the best chat protocol so far. Also WhatsApp, signal, google chat, Facebook are all using it. Plus on your server you can set omemo and opengpg encryption.
Ah I didn't know that Signal used XMPP. It was also called "jabber" before. Anyhow, it's just an event bus system, like Nostr, with a narrow band not broadcast. There's two ways to do multi party, one is with a relay, the other is using multicast. Really they are just specific cases of network protocol. One could easily extend Nostr to do both types of messaging.