There's no guarantee that all the older apps from the Windows 9x/XP days will work today, as some apps back then, especially games, made use of non-public/undoc

There's no guarantee that all the older apps from the Windows 9x/XP days will work today, as some apps back then, especially games, made use of non-public/undocumented APIs or just straight up hacked the OS with various hooks for the sake of performance optimizations. Those are the apps guarantee not to work today even if you turn on compatibility mode.