To be honest I feel like this is throwing the baby out with the bathwater somewhat. If you're writing a script for a system without bash you'll almost certainly know it, so you might as well just put bash in your shebang. I spent a couple of years using #!/bin/sh and avoiding bashisms, but it was literally never useful so I just went back to using #!/usr/bin/env bash.

Of course it goes without saying that you shouldn't use #!/bin/sh then use bash...