Not only is it not more portable, POSIX doesn't even require that it exists at all.
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.
I don't know how this trend started but now it's been cargo-culted to death. Same with the /usr/bin/env thing in shebangs.
One use-case for /bin/sh over /bin/bash these days is that the alpine docker image (base image for a lot of docker images) has /bin/sh but not /bin/bash
