Not every shell script has to be that portable. And even then, "POSIX compatible" shells vary in their "POSIXness". Sometimes using arrays is the right thing to do, or having a reliable indicator of the source script location, so you code to Bash 3. Sometimes associative arrays or something else is better, and your platform is known, so you code to Bash 4 or 5. Maybe all your users are Zsh users so you use that. So you can start by writing POSIX compatible scripts, but there's no sense in tying your hands if you don't need to.
The Bash manual also covers POSIX semantics, I just remember which is which. You're right that the Dash manual is a good place to check what is mostly POSIX (Dash isn't actually strictly POSIX). I would probably use Shellcheck with the correct shebang to double check what's compatible.
The Bash manual also covers POSIX semantics, I just remember which is which. You're right that the Dash manual is a good place to check what is mostly POSIX (Dash isn't actually strictly POSIX). I would probably use Shellcheck with the correct shebang to double check what's compatible.