I host a small Discord for devs hacking on cool tech through my company (for an invite go to https://discord.multiprocess.io). Folks here interested in compilers/databases/emulators/big data/ML are welcome to join, introduce yourself and hang out.
I don't really know what to say. I think Discord is more mature than Matrix, though it's great as an IRC(-bridge) client. You're welcome to start your own community on Matrix if you'd like!