However, a data-ciphers field is missing in gnome-control-center, forcing users to add the data-ciphers field in the NM connection file in /etc/NetworkManager/system=connections manually.
There is a fix in NetworkManager-openvpn that copies cipher to data-ciphers; however, the GUI should accommodate the new option.
gnome-control-center shows the GTK UI provided by the VPN plugin (the same UI is shown by nm-connection-editor).
First of all, you would need a version of nm-openvpn which has support for data-ciphers (no release exists yet, with that feature). Then, nm-openvpn plugin needs to expose the field in the GUI. Which it currently does not.
This seems a rather esoteric feature, especially since the user can just use the cipher option, which is in the GUI. What’s the problem with setting the cipher option in nm-openvpn? Sure, you cannot add multiple data-ciphers, but do you need that? What is the problem with setting cipher in the UI? Maybe that problem should be solved, instead of adding another UI field.