问题
Does implementing the new Sign in With Apple feature make an app incompatible with iOS 12 and below, similar to SwiftUI? Will it be possible to compile an app which has to import AuthenticationServices on something like XCode 10?
回答1:
Sign in With Apple itself doesn't make the whole app incompatible with iOS 12 and lower (the same is for SwiftUI).
But when you need to support this kind of feature (which available on iOS 13 only) you'll need to use @available(iOS 13.0, *) attribute and if #available(iOS 13.0, *) condition to disable support of those features for older versions of iOS. Otherwise you'll have compilation errors and/or crashes on devices running older versions of iOS.
If you want to support Sign in With Apple on older versions of iOS (as well as non-iOS platform) you can use use Sign in with Apple JS framework.
For you second question, as @Paulw11 mentioned, the answer is No, since Xcode 10 doesn't include iOS 13 SDK.
来源：https://stackoverflow.com/questions/57928420/will-sign-in-with-apple-allow-apps-to-be-backward-compatible-with-ios-12-and-l