The iPhone Fold is rumored to have two different types of camera tech for its folding and external displays, as well as have a Touch ID implementation under the screen for biometric authentication.

Apple is continuing to research under-screen fingerprint sensor technology for an iPhone with a view to re-introducing Touch ID in its first ...

A report from Mark Gurman's Power On newsletter says Touch ID won't return to the iPhone any time soon.

This seems to imply that the timeline of under-screen Touch ID hinges on the success of under-screen Face ID.