https://github.com/shellspec

https://oils.pub/

There's probably more.

The shell has a ____wide____ userbase with many kinds of users. Depending on your goal, the rabbit hole can go very deep (how portable across interpreters, how dependant on other binaries, how early can it work in a bootstrap scenario, etc).

These are mine:

https://github.com/alganet/coral

https://github.com/alganet/shell-versions

https://github.com/Mosai/workshop