We had the source code from Interactive Systems for their 386 implementations of IS/1 (pretty bare bones SysV)

Back around 1988.   I had it running on the PC and also was porting it to an Multibus II system (message passing coprocessor).

Anyhow you didn’t need the source to rebuild the kernels and write device drivers.   The necessary header file were there along

with the .o files needed to link your new stuff to the kernel.