How to make root volume writeable again in Catalina?
You can create a symbolic in root directory via synthetic.conf, as an example see below the creation of a symbolic link named bar at /, which points to System/Volumes/Data/bar, a writeable location at the root of the data volume.

echo -e 'bar\tSystem/Volumes/Data/bar' | sudo tee -a /etc/synthetic.conf


After executing the command above, you need to reboot to see effects.

SEE ALSO

man synthetic.conf