In a current project, at least phone or email should be specified in the form. I have declared both fields as mandatory. If only one is filled, then the other must become optional.
In this case the field may only be derequired derequireField($field); but not deactivated $field.prop('disabled', true); in hideField = function(fieldMarker, $form)
For this purpose, there should be an additional action "derequire only" in addition to the existing actions "show" and "hide".