XMPP for reliable, lightweight, & stable. SimpleX is a project worth keeping tabs on as well.

In practice Matrix isn’t decentralized in the slightest. Almost every account is on Matrix.org or a server they host. The whole protocol is a data/metadata syncing machine that isn’t good for privacy & is prohibitively expensive to run anything beyond a single-instance since all history of all users in all rooms necessarily needs to be synced onto the server. Many medium-sized servers have shutdown on storage costs & system resource requirements (especially RAM)—which forced its users often to flock back to Matrix.org. This is wild since it is mostly text chat.

Luckily there are actually decentralized chat alternatives with low system requirements to encourage self-hosting, but man is Matrix so overhyped & misunderstood.