The Computer History Museum has just announced the release of the source code of the original version of Adobe PostScript (https://info.computerhistory.org/aoc-postscript). It is not open source under the OSI definition, however; there is a license agreement that must be signed to get access to the code, and (among other stipulations) the license only allows non-commercial use, and I don't believe it can be re-uploaded to another site such as GitHub.