If you like /bin/bash on a Mac, but are tired of this message.
$ /bin/bash The default interactive shell is now zsh. To update your account to use zsh, please run `chsh -s /bin/zsh`. For more details, please visit https://support.apple.com/kb/HT208050.
Then you need to upgrade bash on your Mac. To be honest, the version that ships is quite old (3.2) and really should not be used.
The correct thing to do is to install a new version of bash on your system.... which you can do by downloading the Bash source, then compiling.
Steps to do this are as follows:
cd bash-5.2
./configure --prefix=/usr/local/bash
make
make install
(as root)ln -s /usr/local/bash/bin/bash /usr/local/bin/bash
ln -s /usr/local/bash/bin/bashbug /usr/local/bin/bashbug