I'm not sure if it's mandatory now either. The sign up button on the front page of Twitter.com says "Sign up with a phone number of email address". If you click on it, it defaults to a form with a name, phone number and date of birth, but there's a link titled "Use email instead" that changes it to name/email/date of birth.

As far as I remember, I signed up without a phone number, and got immediately locked out. I could get access back by providing my number.

I wrote to support that I would rather not use Twitter than giving my number. They unlocked my account.

They’ve required a phone number every time I’ve tried to sign up. They don’t have it in the sign up flow - they wait until you try to sign in and won’t let you proceed without it.