[Hamuko]

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.

[troad]

It’s effectively mandatory. If you register an account without it, you will be banned within fifteen minutes, even if you do absolutely nothing, for “suspicious activity”. The only recourse? Handing over your phone number. Apparently that absolves you of any “suspicious activity”.

Give it a go in an incognito tab now, you’ll see. It’s intentionally deceptive, and they’ve shown time and time again they can’t be trusted with the data.

[i67vw3]

I just signed up recently with email (deleted the account after my work was done), it asks you phone number after 5 minutes of using it.

It blocks you from doing anything unless and until you give them your phone number.