So some people, companies even, have guidelines that describe how to write shell scripts, or even unit tests for shell scripts, as if “UNIX Shell” was a programming language.

That’s wrong.

“Modern Shells” are based on a language that has been written without a formal language specification. The source looked like this, because somebody didn’t like C and wanted Algol, abusing the preprocessor. The original functionality and language rules had to be reverse engineered from that source, and original shell has a lot of weird rules and quirks:

You can use the caret, ‘^’, as replacement for the pipe symbol, ‘|’.

Check out the section

Consider a variable which has been picked up by the shell from the environment at startup. Modifying this variable creates a local copy.

in that document, especially the part where they explain this:

If you call a script directly from a bourne shell (“./script” without shebang), then the shell only forks off a subhell and reads in the script.

The split between original and local copy of the variable is still present in the subshell. But if the script is a real executable with #! magic, or if another sh is called, then fork and exec is used and only the original unmodified variable will be visible.

And it gets better if you go down the entirety of that particular document. If you think Unix Shell is a survivable programming environment, good luck, and please take your code with you while you leave.