[wusspuss]

How much worse than XMPP is Matrix in this regard? On XMPP your and your interlocutor's server can see who you talk to

[zaik]

Not a lot better, but in public MUCs your JID is hidden to everyone except the server the MUC is hosted on. Also MUCs can be configured to only send a limited amount of history to new participants.

For this reason I'm somewhat uncomfortable joining Matrix rooms over bridges, since it leaks my XMPP address and participation to everyone forever.