So unfortunate VMS was never open sourced like Solaris was. It was really interesting, and deserved a fate other than obscurity. Would be cool if zOS was open sourced, but on the other hand the various cloud control planes seem like the "real" OS nowadays in many settings.
DEC used to ship source listings to customers. Not in a form you could compile - a CD-ROM full of PDFs. Although not freely redistributable, there are ISO files floating around the Internet
For MVS, the old public domain versions came with some source code, but it doesn’t match up well with the binaries (the supplied source is an older version missing many patches the binaries have). Newer copyrighted versions they made source listings available on microfiche-it would be great if someone who still has those microfiche were to scan them, although I think people are hesitant to do so since they worry IBM might react negatively (they are copyrighted and possibly under NDA too). And then they stopped issuing microfiche, but apparently they replaced it with some system that IBM customers can log in to and look at some of the source code file-by-file but not download it-I imagine they log every file you look at and if you look at too much they might ask you what you are doing. I know other vendors have similar systems-e.g. Oracle has such a system to let their ERP customers look at some of their ERP source code. (Or at least they did when I worked there years ago-I assume they still do.)
MVS and z/OS always had another issue - large chunks of them are written in a proprietary programming language (PL/S or its successor PL/X) for which IBM has never released a compiler. Well, there was a period many years ago when IBM was licensing their PL/X compiler to ISV’s relatively cheaply, but that didn’t last long. Using a proprietary language with no publicly available compiler helped discourage customers from modifying the source code they received, and also helped discourage would-be cloners, especially in the early days when the idea of software copyright was still very new and hadn’t yet been extensively tested in court.
I also have heard they still ship some source for JES2, a bit of an exception to the general rule of “object code only” (OCO).
"Although NT was not an exact clone of Cutler's previous operating systems, DEC engineers almost immediately noticed the internal similarities. Parts of VAX/VMS Internals and Data Structures, published by Digital Press, accurately describe Windows NT internals using VMS terms."
