For example, 4.4BSD, early Solaris have just about everything you would expect in a modern operating system. And give the age of those systems, they were written in early versions of the C standard.
The C standard has notes about a freestanding implementation, mainly for developing kernels. The requirements C has for freestanding environments are very limited ( http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf , you get float.h, iso646.h, limits.h, stdalign.h, stdarg.h, stdbool.h, stddef.h, stdint.h, and stdnoreturn.h) and the starting point for the program is implementation defined; you might get _Atomic, but that’s optional). For the record, C++ goes a little overboard and promises all sorts of things in its freestanding environment.
https://groups.google.com/g/comp.std.c/c/zzyii-DlMiU/m/RnH5i...
The issue was, do modern operating systems require newer versions of the C standard? Or can you write everything in C89.
Same applies to using C in hardware that lacked memory mapped IO.