I'm well aware of how old the version of Bash is that ships with MacOS and Apple's public deprecation plans. That said, I don't like having to fiddle with this in VS Code and every other tool that tries to be clever. This will kill the message everywhere for processes of this user. Don't bother screwing around with $HOME .bashrc .profiile .bash_profile BS. Or do, but realize the local config specific to the user won't suffice for VS Code et al. You can also use Brew to do this, which will use the most recent version of Bash, but I have a love-hate relationship with Brew and prefer to use this approach instead. Caveat Emptor: This works for me, but use at your own risk.
First, re-enable Bash as the default:
Settings > Users & Groups > cmd-click/right-click on the user > Advanced options > Login Shell > /bin/bash
Edit the profile and bashrc system-wide files
sudo nano /etc/profile
sudo nano /etc/bashrc
Add:
export BASH_SILENCE_DEPRECATION_WARNING=1 to those files.
Close and reopen any open terminal windows or reboot for the change to take effect.