Apple's guidance in the Human Interface Guidelines has always been: "In general, avoid offering an app-specific setting for opting in to biometric authentication. People enable biometric authentication at the system level, so presenting an in-app setting is redundant and could be confusing."
However, FaceID and TouchID behave differently.
With FaceID, a user may configure whether to enable FaceID on a per app basis in system settings, so an in-app setting is redundant and potentially confusing.
With TouchID, a user cannot configure whether to enable TouchID on a per app basis in system settings.
What is Apple's recommendation of a UX for allowing a user to enable TouchID (to log in to the app) on a per app basis. Is the developer expected to provide an in-app setting for TouchID but not FaceID?