I'm pretty sure Windows 7 compatibility mode just makes it so that when a program asks "What version of Windows am I running on?" the system will lie and say "W

I'm pretty sure Windows 7 compatibility mode just makes it so that when a program asks "What version of Windows am I running on?" the system will lie and say "Windows 7." The rest of the compatibility comes from shims written to patch popular and broken applications that incorrectly used Windows APIs in incorrect ways or expected different results from specific calls. Patching broken DRM is beyond what the backwards compatibility can do. SecureROM for instance had a low-level DLL in system32 and games that were "secured" with that DRM used it to take advantage of the system. I read this as that Microsoft isn't providing a way to install that DLL anymore and it would potentially open up an attack vector if they allowed it.