zsh: Not Yet
One reason for this appears to be
bash switching to GPL V3 a while back.
Whatever the reason, though, they are flagging this change up pretty heavily
every time you open a shell:
The default interactive shell is now zsh. To update your account to use zsh, please run `chsh -s /bin/zsh`. For more details, please visit https://support.apple.com/kb/HT208050.
Switching to a new shell will be something of a major project, so although I
will probably do that at some point I was delighted to find that you can disable
the warning by adding the following to your