Zsh becomes the default shell on macOS Catalina
Starting with the macOS Catalina beta, your Mac uses zsh as the default login shell and interactive shell. You can make zsh the default in earlier versions of macOS as well.
Source: Use zsh as the default shell on your Mac.
The reason for the switch away from (a very old version of) bash has to do with licensing.
Time for me to read The Z Shell Manual.