Agreed. If you don't test your script in other shells and on other platforms, why advertise compatibility. And not just the script, but also the programs called.

I got burned by supposedly "portable" scripts by the whole Ubuntu dash disaster, and again when I started using Mac OS X, and then again once I used Cygwin and msys2 on Windows.

I do keep portability in mind when writing shell scripts to ease porting later, but without testing there's really no way to be sure "/bin/sh" is right. And some of the Bash features such as arrays are legitimately useful.