According to the interwebs, zsh became default with Catalina in 2019 [1]; ten years after bash 4 was released with gpl v3 or later.
Also, the interwebs suggest Apple used to use tcsh as the default shell[2]; I don't know when they changed that, but it may have been after bash 4 released? (Thanks, 10.3 was in 2003, so several years before the license changed)