Twitter will no longer offer text messages as a two-factor authentication (2FA) method for all users starting on March 20th. The policy change was announced on Friday and will only be available for paid subscribers of Twitter Blue.
2FA Requirement for Twitter Users
Two-factor authentication is meant to increase account security by requiring a second form of authentication and a password. Twitter has offered 2FA by text message, authentication app, and security key. The policy change was made due to abuse by "bad actors," according to a recent blog post.

Twitter Blue Subscription
Twitter Blue subscription is now open to anyone willing to pay. The subscription fee for Android users will be $11 per month, the same as for iOS subscribers. The blue checkmark, previously only available for verified accounts of politicians, famous personalities, journalists, and other public figures, is now available to all subscribers.
Elon Musk Comments on Policy Change
In response to a tweet, Twitter owner Elon Musk confirmed that the policy change was made because Telcos used bot accounts to pump 2FA SMS, and the company was losing $60 million a year on scam SMS.