Discord already has a huge problem with spam as-is. Yes, it may suck to be randomly locked out of your account when you haven't done anything wrong, but if they removed the requirement altogether it would just open the floodgates even more. Even if you don't trust them fully, they're not some random shady website that's likely to leak your number or use it for advertising. I've been regularly providing my phone number to big services like Discord for years and have never once received spam calls or texts for them. In the incredibly rare case that I do get spam it's always some Hungarian number rather than a foreign one, and they probably just got my number from some public database. It's much more common to get spam via email instead.

Either way, it's pretty easy to get a burner SIM or use Google Voice (if you're in the US), or services like getsmscode.com which provide disposable phone numbers that work with hundreds of major services for 10 cents per SMS, and you can even pay with crypto.