A 1993 version of FREECELL.EXE, part of Win32s, runs unmodified and fully functional on Windows 11, making it one of the oldest GUI applications still natively supported. Earlier attempts to run pre-1993 Windows NT programs faced compatibility issues due to outdated executable formats and calling conventions, but a 1992 CDPLAYER.EXE from NT 3.1 build 347 can also run with some setup, such as CD-ROM emulation. While true 16-bit Windows applications from the 1980s require emulation tools like otvdm to run on Windows 11, the presence of native support for early 1990s 32-bit apps highlights Windows’ backward compatibility limits and achievements.
https://blog.enlyth.com/posts/oldest-windows-gui-app-win-11/