Display Postscript was used in NeXT's NeXTSTEP and OPENSTEP as well as the initial release of Apple's Mac OS X Server 1.0 (Rhapsody), but for OS X proper, Apple switched to Quartz 2D which is closer to PDF than Postscript.

EDIT: And X11 first appeared in 1984 and wasn't released in current form until September 1987, NeXT's partnership with Adobe created their version of Display Postscript in 1987 (the first version appeared in 1986), so X11 didn't have a long history as a protocol at that point.