Can a Windows 10 app built on the Universal Windows Platform (UWP) be ported back to Windows 7 customers? In particular, one made using XAML?
The UWP platform is only available for Windows 10 devices. We can't port it back.
If you want to use it on a Windows 7 device, you can make a WPF prject, which uses XAML, the same as UWP. WPF can run on Window 7. In particular, the XAML and C# code can be reused.