Such a tool does exist, it's called shellcheck. If you give /bin/sh shebang for example it will tell you off using non-POSIX features regardless of whether they'll work with the sh on your system.

(Personally I typically use bash though, largely so I can `set -eEuo pipefail` next.)

The thought is very compelling! shellcheck is pretty great, but of course isn’t this complete. It definitely can’t reticulate splines, for instance.