Twitter settled a lawsuit about this, there was a period where you could sign up without one but your account would be pretty immediately flagged for 'bot like activity' and asked for a phone number to confirm your humanity. They promised to use this for verification purposes only but of course used it for targeted marketing purposes.

> The Complaint alleged that, from May 2013 through September 2019, Twitter encouraged its users to disclose their phone numbers and email addresses for security purposes, such as enabling two-factor authentication and establishing a method for recovering lost passwords. More than 140 million users provided their information to Twitter.

https://www.arnoldporter.com/en/perspectives/blogs/enforceme...