I’ve got a Discord account, on a lot of different channels for FLOSS and other things, and I’ve never set up a phone number. I have occasionally come across certain channels that I can’t join without one, but the vast majority I’ve joined don’t seem to require it
Not to defend Discord, by the way. It’s fucking terrible and I despise this trend of telling people to come to your little private clubhouse to learn more about your software so I can sort through a bunch of obnoxious gif and image spam, while using an absolutely terrible search engine.