It wasn’t really an extension… The nextstep user interface was just written in postscript and objective-C.

This is how the nextstep UI was so smooth and sharp and so well built for design, especially output to print. And how that translated to macOS (although the graphics display subsystem and macOS is based on the more advanced PDF, not postscript). It’s also part of the entire print sub systems, and how it was always so easy to create a PDF in the manner the OP suggests. That functionality was really only added to other operating systems later with plugins.