alt Hacker NewsDiscord has an account flag that triggers a mandatory phone number verification. It happens if you do things like send messages too quickly over the span of about a minute, or send multiple friend requests, or join too many servers, or start too many DMs, or indeed, join any server that is set to require phone number verification.
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.