It's true that bash can be installed on any Unix-like OS, but so can Python, Perl, etc, but it's not available by default, on OpenBSD there's pdksh, FreeBSD tcs

It's true that bash can be installed on any Unix-like OS, but so can Python, Perl, etc, but it's not available by default, on OpenBSD there's pdksh, FreeBSD tcsh iirc, and on MacOS I believe the version of bash is really old due to some previous license change. Edit: I read your post quickly, you addressed the old bash scenario, but still in my opinion I would always, and do target POSIX shell for maximum cross-platform compatibility and that is definitely available on Unix-like OSs by default.