Apple shipped an "old" version of bash due to licensing issues which means if you're relying on any bash 5 extensions you're SOL. Now that zsh is the default is Apple even shipping bash anymore? Things get even more fun if you've multiple versions of bash installed at once.
Yes, they still ship bash because 99.9% of shell scripts have bash in their shebang.
Backward compatibility is king.
It would make a superb April fool's if done correctly :)