The worst part is that they have some sort of FAQ that asks "what if I don't have a phone/don't want to use my phone number" and Discord's official answer is use someone else's. And although I've never been burned by this, I've seen plenty of anecdotes where changing the phone number in account settings can mess up access to the account, or someone can't make their own account with their own phone number because a friend used that number previously.

To me, that's proof that they want phone numbers for mining purposes more than verification ones.