You can't do it as a runtime check. You have to compile the software with the flags to import and link to the latest windows library. The program will hard crash if run on an older version.
I just tried to use the API. I literally couldn't find a working example on the internet in 2 hours so I guess noone figured it out in a decade.
You won't have the chance to call anything. If the executable is statistically linked to the latest API, the executable won't be able to load on older systems.
I'm surprised that these new IsWindows...() functions claim compatibility down to Windows 2000. Doubt microsoft knew how they would name their next release 20 years ago. A Google search indicates that they are in fact broken.
You won't be able to compile with the function imported using the modern lib (because it doesn't exist on XP) but you can conditionally link it at runtime with LoadLibrary/GetProcAddress.
If IsWindows... doesn't work then you can use the VerifyVersionInfo function directly.
Why did they have to narrow it down to games? Could just go with "Saved config (and data)", so other apps could just dump small config files which can be effectively backed up.
A game's exe dir is a long-lived but potentially replaceable thing, like a car. Saved games may be persisted across game versions, so you don't necessarily want them to be in the same directory. Admittedly not the best analogy, but was just observing that it's strange and incongruent to store the 2 together.
Finding the directory requires new APIs introduced in Vista and Server 2008. See SHGetKnownFolderPath