Will the game run on Windows 8.1? The recommended system specs only list Windows 10, I wonder why this would be required.

I took that 30€ gamble, and as it turns out, it seems that the game runs just fine on Windows 8.1. No errors or warning or any sort of technical problems after finishing tutorial and first mission.

the main reason will be that Windows 8.1 is finished so they can't support it... as for the game itself, I would assume single player to work but co-op or any online settings not to sync correct because you really are using the Steam system at that point

p.s, this game is hard with only about 10% of what you need to know covered by the tutorial so have fun learning ;)