As for the metadata leaking, while metadata is obviously available to the admins of the servers you and you recipient are using, these chat histories are not synced in their entirely,

Maybe so, but for a public room it really means nothing because they could just join it anyway. Every client has a copy. The point is neither system has deniability in terms of “I was never talking to this person”. I do think there is more utility in Matrix’s future with P2P accounts however, that don’t depend on a single Matrix server and can be rotated. Anything you aim to be anonymous with should be regularly rotating accounts as we suggest. Take a look at XMPP: Admin-in-the-middle. Admins can get more than enough.

SimpleX chat addresses most of Matrix and XMPP’s shortcomings

Except there is no desktop client, and I’m not sure how it will work at scale. It does not have anywhere near the feature set of Matrix. The whole “spaces” thing is the beginning and I suspect they’ll be doing a lot more there, specifically: “Spaces effectively gives us a way of creating a global decentralised filesystem hierarchy on top of Matrix”.

I hope it can one day replace them.

I honestly doubt that will ever happen they aren’t really competing products. Matrix is really meant for large scale networks, a bit like a whole social media platform, whereas SimpleX is more like a competitor to Signal or Session.

I would like to see Decentralised user accounts and I think they may be still looking at this because it would be nice to be able import your account somewhere else if a home server you’re on shuts down or something.