Lists of earlier versions of Windows to run modern apps:-
1. Windows 9.x - Kernel 9.x, to run Windows 2000, or Windows XP apps, or the apps compatible with versions of Windows, newer than Windows XP.
2. Windows 2000 - fcwin2k, by @blackwingcat, to run Windows XP apps, or newer than Windows XP.
3. Windows XP - OneCore API, by @Skulltrail, to run Windows Vista, or Windows 7 apps, or newer than Windows 7.
4. Windows Vista - Patched kernel files, by @win32, to run Windows 7 apps, or newer.
5. Windows 7 - VxKex, by @vxiiduu, to run Windows 8, or Windows 8.1 apps, or newer.
6. Windows 8, or Windows 8.1 - Based on Windows 10 build 9888, by me, to run apps compatible with Windows 10 or newer.