Epic Games originally announced on September 9 that Fortnite was losing its Sign In with Apple feature on September 11. The said date came, and Epic released an update saying Apple is giving them an "indefinite extension" to use it. The developer still recommends preparing for its removal by signing in with an email address and password instead of relying on Sign In with Apple. The feature is similar to what Google and Facebook offer, where you're allowed to register/sign in to an account or service using your Apple ID. Apple promises it offers greater privacy with options like Hide My Email.
It isn't clear why Apple decided to extend Epic's access to the feature given how their legal fight continues to escalate.
Source: MacRumors