This is a bit of a Linux specific view of the universe. Yes posix has a standard for how sh should work. But there were specific implementations of sh, and you

This is a bit of a Linux specific view of the universe. Yes posix has a standard for how sh should work. But there were specific implementations of sh, and you would get those on commercial Unix or *BSD. Later shells came in and filled some gaps. OpenBSD still has ksh by default. Linux distros had bash as their /bin/sh.