I've got a couple dozen accounts registered between defunct bots that were tied to irc bots or blogs, and single-use numbered accounts registered for contests and giveaways that required twitter. None of them has ever required any proof or phone number for registration. My primary account, which I use just as infrequently? Of course they asked for my phone number. I'm just annoyed I can't do 2fa without the twitter client or sms.

For Twitter: I believe that's just part of their signup flow

Likely significant reduces the bounce rate compared to requiring a phone number while creating the account

Same here. I let it stay suspended. Weeks later it came out that Twitter had a flaw that let anyone see your phone number.

It will be a cold day in hell before I give them my number, and I survived the Texas icepocolypse this year.