Since Qt 5.7 the WinRT position plug-in can work on Win81 and Win10 desktop platforms too. Older Windows platforms are excluded due to the non-existence of the relevant Windows APIs.
Currently, we do not build the plugin for windows desktop though. The plugin should be enabled on those Windows targets where the Windows SDK/compiler can build it. Non-supported platforms will fail during the plugin loading which is caught by QGeoPositionInfoSource already though.