If you invoke the bash shell while macOS Catalina is configured to use a different shell, you'll see a message that the default interactive shell is now zsh. To silence this warning add to /etc/profile for system-wide effect:

Bash Shell - Silence macOS Deprecation Warning

# System-wide .profile for sh(1)
export BASH_SILENCE_DEPRECATION_WARNING=1

if [ -x /usr/libexec/path_helper ]; then
    eval `/usr/libexec/path_helper -s`
fi

if [ "${BASH-no}" != "no" ]; then
    [ -r /etc/bashrc ] && . /etc/bashrc
fi