Can a custom keyboard app extension detect a specific user’s identity based on settings/credentials/identification mechanism in the companion app or otherwise? By "identifying a specific user", I mean in the context of my own application/service/brand, not personal/phone data on their device.
Or is it always a static keyboard, always unable to determine the user/phone using it?
I think you should be able to. In your companion app, put whatever identifier you want in the NSUserDefaults, and let the keyboard pick that stuff up in its viewDidLoad of its UIInputViewController subclass.
Let me know if that works.