Windows games for versions of Windows from Windows 95 to Windows 2000/NT.
For PC games before Win95, pretty much everything can be emulated properly with DOS PC emulators. And from Win7 on, the software and hardware standards had mostly restabilized and WINE/Proton usually works well. But in between, there was a real Wild West of third-party libraries and official patches that were constantly being updated with compatibility-breaking changes. You can go through the hassle of setting up a whole virtual machine that really taxes your computer and still not get some games to work.
Most recomps seem to be of popular console games that already run quite well through easy-to-use emulators. It'd be nice to see more recomp efforts for less-well-known games that are harder to play at all on modern hardware. I know it's unlikely to happen, though; who wants to spend their efforts on niche games that most people haven't heard of, much less care about? Only someone who has a personal interest in that game.