Quake is a rare exception. Source availability is rare on Windows, falling to almost zero for commercial applications (for obvious reasons). There's also plenty of corporate internal applications where the one company that is using it is also the only one with the source .. and they've lost it.
Quite a lot of game source is lost entirely even by the original authors.
Not to mention that even if you do have the source, changing the use of an API can be a really expensive software modification project. Even Microsoft haven't been entirely systematic, you can easily find WinForms control panel dialogs in Win11.
Some embedded Windows apps exist in this space as well. Oscilloscopes and other expensive scientific instruments that run Windows XP.
Quite a lot of game source is lost entirely even by the original authors.
Not to mention that even if you do have the source, changing the use of an API can be a really expensive software modification project. Even Microsoft haven't been entirely systematic, you can easily find WinForms control panel dialogs in Win11.
Some embedded Windows apps exist in this space as well. Oscilloscopes and other expensive scientific instruments that run Windows XP.