Once T401774: Redesign UI for listing 2FA methods and adding new methods is done, users will be able to delete individual 2FA methods directly from Special:AccountSecurity. When a user tries to delete a key, they already have to click through a warning explaining that they won't be able to use that key anymore.
However, when a user tries to delete their only remaining 2FA key, we should display a different or additional warning explaining that they're turning off 2FA completely.



