Currently 25 states require it. Incidentally 23 of to lowest 25 states allow child marriage. Funny isn't it?

From https://www.theverge.com/news/797051/discord-government-ids-leaked-data-breach :

Of the accounts impacted globally, we have identified approximately 70,000 users that may have had government-ID photos exposed, which our vendor used to review age-related appeals.

So they only demand your ID if they've somehow decided that you're underage and you want to appeal that.