"Servers" can set only phone verified users to be able to join. Depending on your usage of Discord you may have never joined a server that has thet turned on.
Even outside such "servers", discord may or may not demand a phone number based on some kind of trust score. Get reported a lot -> better have a phone. Happen to get an IP previously used by someone who got reported a lot -> better have a phone. Use an uncommon browser version -> likely going to need a phone number. Have a slow connection during signup -> gib phone number "to show you are not a bot". Like any such scheme, it barely slows down determined actors since many (especially mobile) ISPs hand out new IP addresses like candy. But I bet it looks good on reports about how many evil users they stopped and lets not think too much about legit users like gp getting caught up in this - those are a small enough fraction that Discord can ignore them without consequence.