I am in dozens of servers and have not encountered this demand for a phone number. I have been in servers that required it for moderators as part of 2FA, and I just declined to moderate there. It had no effect on my use of any other server.
It has happened to me on two accounts. OP is also not the only other person I've seen who has dealt with it.
Bully for you that you haven't encountered it, but it's certainly a thing.
Anyone on HN should know the fickle nature of fraud detection, especially when the cost of getting it wrong is 0.
Just because something hasn't happened to you, doesn't mean it doesn't happen to other people