Skip Signal and move to Matrix before it's too late https://digital-justice.com/articles/skip-signal.html przez @wallabagapp
The "metadata" you called is: list of events, list or room members.
That "metadata" shared across room's participants servers.
Do you know what protocol uses pretty same approach? XMPP. Do you know why? Because without that info you can't federate. Yes, even in XMPP.
Of course, matrix and XMPP are different things, but federation concept is pretty same.