When we enabled the capability for our app for Sign in with Apple in our App ID Configuration, it caused a provisioning profile failure in app versions that do not support Sign in with Apple - specifically iOS 12. So how do we continue developing and releasing for iOS 12 while we develop and test Sign in with Apple for iOS 13?
Testing Sign in with Apple while supporting iOS 12
Have you looked into feature bifurcation based on iOS version? I believe there is an idiom for that that when coupled with an if would allow your app to handle both examples.
You may want to wait for that feature to be released, tho, to see if Apple provides for you.