Matrix is probably the most popular federated alternative to Discord out there right now. A lot of open source projects are now adopting it as their chat platform of choice and over the past few years it’s really started to gain momentum. They have a good dev team as well.

Matrix also has really cool features like E2E encryption for chat rooms and DMs. There’s also the concept of Spaces, which are like Discord servers but arguably more flexible.

I highly recommend checking out Element, which is the flagship client made by the guys driving the Matrix protocol.