Switching to Zsh in macOS Catalina not working
Okay - one more place to check:
In Terminal's Preferences, under the General pane, there's an option to use the default login shell or set another. For some reason, mine is set to use another (probably set this in a far away time and place for some long done learning opportunity, lol).
Selected "Default login shell", opened a new Terminal window, and voilà, all is working as expected! Sheesh...
Found to look there in the same support article, under "How to use a different shell without changing the default".