Veteran Windows Insider Xeno has released a 69.0MB version of Windows 7 x86, which can boot but lacks support for most software packages due to severe pruning. This was done as a proof-of-concept, and work is underway to make it more usable for retro enthusiasts. To run even basic programs, users must provide their own system files.
The current release has limited functionality, with critical missing files such as common dialog boxes and common controls. However, Xeno hints that an adjusted version of the OS might be developed with enough core system files to support older games and apps.
Xeno’s goal was to create a “fun proof of concept,” but some enthusiasts are expressing interest in making it more practical. With further pruning efforts, the OS could potentially remain at the same size while still supporting necessary features.
Source: https://www.tomshardware.com/software/windows/veteran-windows-insider-creates-windows-7-install-measuring-just-69mb-in-size-system-boots-but-has-been-pruned-so-severely-virtually-nothing-can-run-for-now