The 3% for Windows 8.1 made me chuckle because I'm one of them.

EDIT: Don't get me wrong, it's not by choice. :P

I'm running Win 10 on all my systems, now.
First was my gaming desktop, immediately at release of Win 10, while I kept Win 8.1 on my laptops.
My gaming laptop followed shortly after, when I was sure that I'd stay with Win 10.
And my oldest laptop finally received the Win 10 upgrade shortly before the possibility for a free upgrade ended (officially).
Haven't looked back since.

As long as it says "Works on: Windows (7, 8, 10) " I assume that this will be the case. I am not sure but it could be that the Win10 reqs are the official requirements made by the devs/publisher and that GOG has tested the game on the other still used and listed OSes and guaranteed it to run on those as well. However with GOGs new refund policy you could always test if it still runs on Win7 or Win8 and if not refund it. I do agree however IF it should still be able to play this on systems other than Win10 it is bad marketing to exclude those systems. My guess is that the devs simply never tested it and therefore wanted to be cautious and only included the system they developed and tested the game on.

And my assumption seems to be correct:
https://steamcommunity.com/app/606230/discussions/0/1736593325992710566/