The bash that comes with macOS is a fork of the last version distributed under version 2 of the GPL, most likely because Apple doesn’t want to distribute GPLv3 code as part of macOS, and it is ancient. They keep it updated with security fixes but nothing else, so it has gradually become less and less compatible with current bash.

Since zsh has become a popular bash replacement and it isn’t GPLv3, they switched the default shell to that.

Bash binary is still included in MacOS, right? It’s just the default terminal shell changed to zsh.

Not sure either, and kali did the same. https://apple.stackexchange.com/questions/361870/what-are-the-practical-differences-between-bash-and-zsh

Of course, someone posted on stackexchange a nicely detailed post.