Element X is a new Matrix app for mobile
If you're not aware, Matrix is a decentralized end-to-end encrypted chat service, pretty much like WhatsApp or Signal. The reason I like Matrix is that it can be truly yours: it's not dependent on a centralized trusted setup owned by one company. Instead, anyone can operate a server (pretty much like e-mail).
For a while now, in order to use Matrix on mobile phones, your best bet was to use Element. It is a fully-featured app, that allows you to chat, make calls, use bots, etc. The only downside is that it's based on a webview (as far as I can tell). So it can be a bit slow.
The new effort by the Element team is Element X, which seems to be a complete rewrite that has lots of things implemented natively, and which shares those native parts between different platforms. The common part now lives in the Rust SDK, that would be a reference implementation of the client from now on.
I've configured the new sliding-sync on my server and Element X connected to it without any major issues (except maybe slow initial history sync). The app feels a lot snappier overall.
I'm really glad the authors are investing into better clients, because for many "normal" people this is the product. And many open source communities will be better off eventually if they're able to replace Discord with something that works well and is visually smooth.