This seems to be default behavior from Unity due to most apps using 3D content and requiring the Metal GPU pipeline. Otherwise, if this hack restores simulators, as well as enabling the Target SDK to Simulator in Other Settings of Player Settings in the Unity Project, the app will produce an exception the moment the 3D scene attempts to render: Metal Submission Thread (50): EXC_BAD_ACCESS The best workflow for testing UI layout on various devices for a 3D app would be Unity's Simulator Window.

The Unity player settings change will only work if the app is 2D, per this post: https://developer.apple.com/forums/thread/12032?answerId=840010022#840010022