For games, XP has some relevancy left if you're targeting internet cafes in China or nearby regions (and maybe some others), but even that is giving over to Windows 7 or newer -- XP is already small, and only going to get smaller. Keep in mind that supporting XP means supporting Direct3D 9 and other legacy APIs, which is an entirely different level of expense than supporting the modern APIs you're already using on down-level operating systems that support them.
Also ask yourself if a PC that meets the hardware requirements of your game is likely to be running XP to begin with -- the number of people running XP on hardware suitable for Windows 7 is pretty small.
I'd say that Windows 10 is now a pretty safe bet -- since Windows 10 was a strict improvement over Windows 8/8.1 there was no reason not to do the free upgrade that Microsoft offered. Add to that that app-store model for Windows 8/8.1 is now effectively deprecated, I'd personally be entirely comfortable assuming Windows 10. If I felt compelled to support down-level operating systems I'd support Windows 7 in addition to 10, there's no compelling technical or market-share argument to support Windows 8 or 8.1.