Twitter lets users switch to 2FA powered only by security keys on both the web and mobile apps. Security keys are the only two-factor authentication (2FA) method while having all other login methods disabled. The social network also upgraded security key support to the WebAuthn standard and enabled the ability to use 2FA on any Twitter account without a phone number. Twitter has first added security keys as one of several 2FA methods on the web in 2018 and support to use security keys when logging into mobile apps for 2FA-enabled accounts in 2020.
Source: https://www.bleepingcomputer.com/news/security/twitter-now-lets-you-use-security-keys-as-the-only-2fa-method/