https://en.wikipedia.org/wiki/System_programming_language
https://en.wikipedia.org/wiki/Category:Systems_programming_l...
Some examples, out of my head.
- Burroughs, now being sold as Unisys ClearCase, used ESPOL, later replaced by NEWP, which already used the concept of UNSAFE code blocks;
- IBM used PL/8 for their RISC research, before switching to C, when they decide to go commercial selling RISC hardware for UNIX workstations
- VAX/VMS used Bliss
- Xerox PARC started their research in BCPL, eventually moved to Mesa (later upgraded to Mesa/Cedar), these languages are the inspiration for Wirth's Modula-2 and Oberon languages
- OS/400, nowadays known as IBM i, was developed in PL/S. New code started to be replaced by C++. It was probably the first OS to use a kernel level JIT with a portable bytecode for its executables.
BitSavers and Archive web sites are full of scanned papers and manuals from these and other systems.
But what were those other OSs and languages? Sounds interesting!