According to the App Store review guidelines update posted today, Sign In with Apple will be required for any iOS app that implements a single-sign in button.
"Sign In with Apple will be available for beta testing this summer. It will be required as an option for users in apps that support third-party sign-in when it is commercially available later this year."
"Sign In with Apple will be available for beta testing this summer. It will be required as an option for users in apps that support third-party sign-in when it is commercially available later this year."
https://developer.apple.com/news/?id=06032019j