This project runs on all shells, totally portable:

https://github.com/alganet/coral

busybox, bash, zsh, dash, you name it. If smells bourne, it runs. Here's the list: https://github.com/alganet/coral/blob/main/test/matrix#L50 (more than 20 years of compatibility, runs even on bash 3)

It's a great litmus test, that many have passed. Let me know when just-bash is able to run it.