Login using should not be shown if it should not be used
When you edit an user from the Admin Console you can change its Login using parametre from the default Modern value to Classic.
I have tested it with the admin user and after login in another browser I did not find any difference. No classic UI shown at all.
I think this option should be removed or probably hidden.
Note: This Carbonio version was installed on 2022-01-13 from an Ubuntu 18.04 minimal installation.