For now, and since you can't make an iPhone 7/Plus-only app for the store (yet?), I think you can only make it an option and code accordingly. If that's the only thing your app does, then you may be stuck unless you can expand it's overall functionality. Only App Review would know if they'd reject/approve, tho.
From the docs:
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Some iOS devices contain a dual camera—two separate back-facing cameras that can work together as a single capture device. When setting up a capture session on a device with an dual camera, you can choose to use only the wide-angle camera, only the telephoto camera, or the dual camera. Choosing the dual camera capture device provides behavior similar to the built-in Camera app—the system automatically chooses which camera to use during capture, and can combine data from both cameras to improve capture output.
The
AVCaptureDevice
devices
and devicesWithMediaType:
methods are deprecated in iOS 10 and do not provide access to the dual camera device (instead, they provide only the wide-angle camera device). To determine whether a device contains a dual camera and select it for capture, you may either:When you use the dual camera capture device, Optical Image Stabilization (OIS), RAW Photo Capture, High Frame Rate (HFR) / Slow Motion Video, and most manual controls are not available. To use these features, specifically select either the wide or telephoto capture device. (For OIS, specifically select the wide-angle camera; the telephoto camera does not support optical image stabilization.)
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Additionally, you may want to file a UIRDC dual camera key request via the bug reporter, adding your report # to your thread for reference.