Export $BASH_SILENCE_DEPRECATION_WARNING as described in the Apple web page pointed to by the nag message, or change your shell to your own version of Bash.

See also <https://apple.stackexchange.com/questions/371997/suppressing...>. I went with the "use an updated brewed Bash" approach, which has been working well. Using `sudo chfn` means you don't need to futz around with editing /etc/shells.