Earlier quoted context omitted.
Most UNIX systems that were not implemented in C, and thus lacked the symbiotic relationship, never survived in the market, sadly. There have been UNIX systems implemented, Pascal, Ada, Modula-2, Modula-3, as the most relevant ones. All gone. Also note that POSIX/UNIX certification requires a C compiler presence.
Linux kernel is also one of the few that do not use C ABI as entry point for user programs at all. As for C compiler presence in POSIX, only existence of C-accessible APIs with specific structure are mandated, C compiler is optional just like Fortran runtime and compiler are.
https://pubs.opengroup.org/onlinepubs/9799919799/nframe.html
https://pubs.opengroup.org/onlinepubs/9699919799.2018edition...
https://pubs.opengroup.org/onlinepubs/015967575/toc.htm
And copying this from UNIX 03, the most widespread certification,
"A single configuration of the system shall meet all of the conformance requirements defined in the following mandatory Product Standards:
Internationalized System Calls and Libraries Extended V3
Commands and Utilities V4
C Language V2
Internationalized Terminal Interfaces
The product must be registered as conformant to the Product Standards prior to, or concurrent with, the UNIX 03 Product Standard registration."