Not having bash installed by default is not the issue here. The issue is that macOS comes with a buggy version of bash and those bugs cause valid scripts to fai

Not having bash installed by default is not the issue here. The issue is that macOS comes with a buggy version of bash and those bugs cause valid scripts to fail in a unique way. So at first you might think everything is just fine (because there is a bash) and but in fact you either have to test against those bugs or install a newer version.