The metadata that Matrix “leaks” is the timing, size, sender and recipients of messages, which is exposed only to the servers participating in a given chatroom.

The metadata that Matrix “leaks” is the timing, size, sender and recipients of messages, which is exposed only to the servers participating in a given chatroom. Same as PGP encrypted email, or OMEMO encrypted XMPP. Matrix also doesn’t encrypt emoji reactions or room key/value data yet. Meanwhile Signal exposes the timing, size, and recipients of messages to the server (complete with their phone number!) - just not the sender, thanks to sealed sender. Whether you consider Matrix’s metadata footprint a privacy problem depends on your threat model. Meanwhile we’re working on P2P Matrix (arewep2pyet.com) to obfuscate the metadata to be better than Signal (given ideally it never needs a server, and if it does, it goes via obfuscated store & forward relays).