Sadly, Apple is requiring us to implement "sign in with apple" if we already have sign in with other providers like facebook or google:
Guideline 4.8 - Design - Sign in with Apple
Your app uses a third-party login service, but does not offer Sign in with Apple. Apps that use a third-party login service for account authentication need to offer Sign in with Apple to users as an equivalent option.
Next Steps
Please revise your app to offer Sign in with Apple as an equivalent option for account authentication.
How does everybody handle this? Is there some xamarin/c# sample or tutorial?