Ah, just re-checked that community and apparently they must have phone-gated it recently, as I haven't seen Blender use that before. Perhaps they got hit with spam, which I know is the reason some add the requirement, even temporarily (which I suppose circles back to the annoying side-effects of spam).

This isn't just a server-specific thing. Discord can and will randomly lock you out of your account completely until you verify with a (unique) phone number even if none of the servers you're in require it. If you have a discord account with no phone number, it will probably happen to you eventually.