Mumble -- For IBM and DEC in the 60s and early 70s, the manufactures distributed the (assembler) sources to the OS and we could (and did) build from source but usually just built parts. By the time of VMS and the other minis, you tended to link together from modules, although many sites did have sources (in assembler).
Remember, the target was the manufacturers HW so they were not giving away much. In the case of IBM, eventually, Amdahl started cloning and they got a tad more closed, but by that time there were also many mainframe OS flavors in wild.
That said, I think Burrough's gave away the ESPOL code for their systems, but I never saw it; so I can not speak definitively there.
Unix was different. Like Burrough's, it was heavily written in a systems programming language. To my knowledge, the 'concept' or 'porting' the OS in it's entirety to a completely new ISA began with UNIX.