There is a new privacy NSFaceIDUsageDescription Info.plist key in the Xcode 9 GM where a developer supplies the usage description for the permissions alert. Does anyone have a link to docs on it? The plist view in Xcode summarizes it as:
Privacy - Face ID Usage Description
Now further explained in the above mentioned forum (by an Apple employee) https://forums.developer.apple.com/message/265156:
Face ID requires adding a usage string with the key NSFaceIDUsageDescription (aka Privacy - Face ID Usage Description) to your app's Info.plist.
In the simulator, the Face ID permissions alert will show "This app was designed to use Touch ID and may not fully support Face ID" if that key is missing. On an actual device, your app will crash the same as if any other required usage string is missing.