SHELL should be there in all environments.
Using “/usr/bin/env bash” is the correct way to find bash, or for that
matter, any interpreter.
I had done work 5-6 years ago to get LE to work on MacOS but have
since then lost the system I worked on to get that going. It was a
work laptop and had to give it back when I left the company.
Certain things don’t work because of differences between BSD and
Linux/GNU tools. I had tried to resolve those and submitted patches.
I’m not sure if those got accepted and were made part of a release.