The instructions for enforcing a default shell for users in macOS Catalina through Active Directory is available at Apple: https://support.apple.com/guide/directory-utility/set-a-unix-active-directory-user-accounts-diru34cb1e36/mac
Here's the procedure to manually update the shell (independently of any AD setting):
If the computer a natively installed with Catalina, then all user accounts will use
If the computer was upgraded to Catalina, then:
If a user wants to change their default shell environment from
bash, the following command can be issued at the command line prompt in Terminal.app:
chsh -s /bin/bash
You can also change the default shell via System Preferences → Users & Groups. First unlock the padlock at the bottom-left, then control-click the user to edit and chose "Advanced options..." from the popup context menu. Edit the Login shell property by selecting the shell from the drop-down control.