Sorry, I meant Mac OS X used to include Windows 95/NT UI artwork (buttons, scrollers, etc) but I think it was purged 10 or 15 years ago.

Can you elaborate on this? Literal Windows UI artwork or Windows-like?

I never knew about this and was a NEXTSTEP/OpenStep/Rhapsody/OS X user and spelunker. Tell me more!

So OpenStep for Windows NT ran under windows NT and it looked like Win32 GDI but it wasn't Win32 GDI. It was still display postscript but the GUI elements (NSButtons, etc) were themed to look like Windows NT instead of NextStep. For whatever reason they were included in Mac OS AppKit as well.

Take a look at https://hetima.github.io/fucking_nsimage_syntax/2.html - specifically NSWin* images (sliders, checkboxes, arrows). You can also see some NextStep era artwork and MacOS 9 (Rhapsody) artwork.