Earlier quoted context omitted.
It's actually more appropriate to install dash, and write the script with that. When this is done, it will run in all the shells, because the script becomes POSIX-compliant.
Making a macOS-specific script which does not make sense to run anywhere else portable to other platforms is entirely a waste of time.
Portability between bash and zsh is a waste of time?
Wasn't a previous argument that legacy Macs should remain supported if possible?
smh.