Twitter will limit the use of text messages for two-factor authentication to only paid subscribers starting March 20, due to abuse by "bad actors." Users who have SMS-based 2-factor authentication enabled and are not Twitter Blue subscribers have a 30-day window to switch to another method.
Elon Musk confirmed the change in response to a user's tweet, stating that the company was losing $60 million annually to scam SMS. The blue checkmark is also now available to anyone who pays.