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What metadata does XMPP leak? AFAIK only when a message was sent, roughly (in large increments) how large the message was, the server of the sender knows from who to which server, the server of the recipient knows from which server to who.
I find it strange that Signal somehow doesn't know when a message was sent, and from who to who; how would they ever make this possible?

Also, you say you have yet to find any other free service that collects as little data... How about most e-mail providers? Not Google and Microsoft of course, but most e-mail providers only need a name which can be made up as well. You hm also host your own email server, then you are in control. All of this is true for XMPP and Matrix, as well.