There will be an arm version of windows.. but there’s no reason to believe that Microsoft will or can support all of the coprocessors that Apple is going to add - including gpu, t2, nueral engine, etc. it isn’t like Apple plays nice with 3rd parties. I seriously doubt they’ll release the details necessary to develop independent drivers for those... whether it’s Microsoft or Linux.
So there’s good reason to believe it may be macOS only.
Personally I’ll believe it when I see it... because right now there are no 3rd party iPad or iPhone operating systems. So a better question is probably: what makes you think there will be 3rd party OS support?