And when it comes to FOSS chat, there's also XMPP, IRC, Zulip, and Rocket Chat (which speaks Matrix, btw). There are a lot of options!
Slack has been squeezing folks for years already, and notably continues to lack features critical for open communities (like, idk, being able to block someone).
I wonder how long till Discord starts putting on the squeeze? Already the interface is chock full of self-promotion and upsells...