In the documentation it states "A capture output that records scene depth information on compatible camera devices". What are those devices?
iOS 11 - Which devices are compatible with AVCaptureDepthDataOutput?
At the moment, depth data output is only supported on the dual-camera iPhone 7 Plus. At least that's what we've seen in our tests so far.
If you want to find out, you can use AVCaptureDevice.DiscoverySession with mediaType: .depthData, but at the moment it only appears to be the 7+ as the depth data is based on stereoscopy, and it's the only device with two cameras.