When Apple released MacOS Catalina, it decided to switch the default command line shell from the Bourne Again Shell, aka bash, to the Z Shell, aka zsh. One reason for this was that Apple installs a rather old version of bash, 3.3.57, to allow it to include the software under a licence it’s happy with. This isn’t a problem that affects zsh, so Apple can bundle a much more recent release.
That was no problem for me, either, because I long ago used Homebrew to install an up-to-date version of bash, 5.0.17, and have been happily using in preference to the Apple one. To do so yourself, run
brew install bash and then go to System Preferences > Users & Groups. Unlock if you need to then right-click on your name in the left-hand column and select Advanced Options…. Now highlight the Login shell: field and set the path to your preferred shell, in this case
/usr/local/bin/bash. Afterwards, you can enter
echo $SHELL to confirm the change.