Informally; there's a few mods of the stock Discord client; however they're all closed source because they cannot possibly be fully open source as the stock Discord client is Closed Source.
Basically; alternatives exist; However they are not FOSS, FLOSS, OS or even fully Source Available.
Beeper exists but this is not strictly a Discord-only client and it is not fully Open Source; as some client/server code is only "Source Available". Beeper is also not fully "Free"; unless you self-host it. Furthermore access to Beeper is heavily join-gated. You will have to source an invitation code from someone you know. (No, I will not provide one.)
If you do get invited to Beeper; use Beeper Cloud, and be aware that iMessage integration does not work.