[pjmlp]

Burroughs B5000 was written with zero lines of Assembly in 1961 with ESPOL, followed by NEWP, both Algol 60 variants with intrinsics.

Xerox PARC used Mesa with microcoded CPUs, after using BCPL initially.

IBM used PL/8 for their RISC OS and compiler research.

OS/400 was written in PL/M.

There are plenty of other examples, UNIX was not the only one to be written in an high level language, regardless of what C devs advocate.