Display Postscript was only used in Rhapsody, the very first releases of what later became OS X, but not in Classic MacOS (up to System 9).
Apple removed Display Postscript for OS X - it was insecure by default, e.g. allowing remote file access, when using the remote display feature on NeXT/OpenStep. I guess Apple also wanted to avoid the Adobe licensing fees....
No, Adobe didn't follow through on providing a free license for Display PostScript as promised --- this killed a lot of software companies which had NeXT/OPENstep software ready to go --- of course, Adobe didn't care (they couldn't even bother to keep track of the source code for Glenn Reid's TouchType.app) and were glad to kill off competition such as PasteUp.app
Mike Paquette and the rest of his team spent a decade re-creating Display PostScript as Quartz, née DisplayPDF.
I'm not sure how you could fail to mention that Apple cleverly avoided Adobe's licensing fees by moving from Display PostScript to Display PDF.[1] PDF was free under GPLv2, and if I'm not mistaken, still is, so Apple shouldn't mind still using it... as long as it doesn't change to GPLv3.
Thanks for adding this - I considered to include it, but couldn't find a source stating that the technology was officially called DisplayPDF.
I think Apple's official term for the OS X graphics layer is "Quartz" and there's a short discussion about the use of PDF in Quartz on Wikipedia which states "Quartz's internal imaging model correlates well with the PDF object graph, making it easy to output PDF to multiple devices":
This arstechnica article linked on the Wikipedia page gives some more details, but it was written in 2000, so many things might have changed since then:
I think it's reasonable to assume that Apple's Quartz code in OS X no longer includes any actual Adobe code, so this might have been sufficient to avoid the licensing fees.
Apple removed Display Postscript for OS X - it was insecure by default, e.g. allowing remote file access, when using the remote display feature on NeXT/OpenStep. I guess Apple also wanted to avoid the Adobe licensing fees....