Apps that let users sign in with a third-party service such as Google or Facebook for account creation are required to also offer Sign in with Apple as an equivalent option, with equivalent visual prominence, with limited exceptions.
Very commonly caught when an app adds "Sign in with Google" or a social login SDK without also adding Sign in with Apple alongside it.
"Your app uses a third-party login service but does not offer Sign in with Apple as an equivalent option, per guideline 4.8."