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.

I've never been in a situation where I had to care. Bash is everywhere, it's a de facto standard of its own. Even a lot of buildroot and Alpine based Linux deployments, which don't come with bash by default, usually have bash added to them.

Eh, bash is pretty damn portable