Apple, at least, maintained a historical database of your phone's notifications, that it did not clean up after they expired. That includes all notifications from Signal telling you that person XXX has sent you a message that starts YYYY <facepalm>
Anyway, I'm not OP, and they have a mad setup (XMPP via Tor) which is a flaky solution most people wouldn't go for. In general, if you're not going to such extreme measures of hiding among the crowd of Tor users to mask your metadata, you're better off directly connecting and hiding among the crowd of Signal users, rather than hosting your own instance.
This is the whole point. Signal actively prevents me from using it outside of the Apple-Google duopoly. Other messaging apps are not like this.