How to make root volume writeable again in Catalina? After the installation of Catalina all User created folders from root are moved into a separate folder and the root folder is made readonly, containing only system default folders. 

How is it possible to undo this and to make the root folder writeable again?
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