Same for me, and my account is almost a decade old. I think it depends a lot where are you from and the kind of activity, as i read stories of people being asked to register a number out of nowhere. Many servers requires you to have it tho, due to spam protection. I just don't talk on those.

Each discord server can decide whether they only will allow people with a phone number on. When you hit one of those, Discord will ask you for your number.