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

"If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well"

ShellCheck can check for POSIX compliance with the option `--shell sh', e.g.

  shellcheck --shell sh myscript.sh

Bash is POSiX compatible.