sh is, but not the location.

> Applications should note that the standard PATH to the shell cannot be assumed to be either /bin/sh or /usr/bin/sh, and should be determined by interrogation of the PATH returned by getconf PATH, ensuring that the returned pathname is an absolute pathname and not a shell built-in.

https://pubs.opengroup.org/onlinepubs/9699919799/utilities/s...

However, in my experience, I don't recall seeing any POSIX or POSIX like system that didn't have /bin/sh.

Oh interesting. I assumed that since even Nix has a hard-coded /bin/sh, it must be required. Probably _essentially_ required.