We continue to be committed to keeping people safe and secure on Twitter, and a primary security tool we offer to keep your account secure is two-factor authentication (2FA). Instead of only entering a password to log in, 2FA requires you to also enter a code or use a security key. This additional step helps make sure that you, and only you, can access your account. To date, we have offered three methods of 2FA: text message, authentication app, and security key.
While historically a popular form of 2FA, unfortunately we have seen phone-number based 2FA be used – and abused – by bad actors. So starting today, we will no longer allow accounts to enroll in the text message/SMS method of 2FA unless they are Twitter Blue subscribers. The availability of text message 2FA for Twitter Blue may vary by country and carrier.
Non-Twitter Blue subscribers that are already enrolled will have 30 days to disable this method and enroll in another. After 20 March 2023, we will no longer permit non-Twitter Blue subscribers to use text messages as a 2FA method. At that time, accounts with text message 2FA still enabled will have it disabled. Disabling text message 2FA does not automatically disassociate your phone number from your Twitter account. If you would like to do so, instructions to update your account phone number are available on our Help Center.
We encourage non-Twitter Blue subscribers to consider using an authentication app or security key method instead. These methods require you to have physical possession of the authentication method and are a great way to ensure your account is secure.
Taken from: https://blog.twitter.com/en_us/topics/product/2023/an-update-on-two-factor-authentication-using-sms-on-twitter