That's only true for NVidia and that's because they don't want to put the work in (They do plan to make it work on Linux though just not on pre WDDM2 Windows).

That's only true for NVidia and that's because they don't want to put the work in (They do plan to make it work on Linux though just not on pre WDDM2 Windows). There is absolutely nothing locking multiGPU support to windows 10 other than the willingness of IHVs to support it.