The Customize menu is to manage Custom fields only, and you can no longer show/hide them from this menu since the toggles have been recently removed. The + button on the right of the columns is just a shortcut to add a custom field.
The ‘Tags’ field is grouped within the 9 default fields such as Assignee, Due date, Projects, Created by, Dependencies etc. that cannot be customized, therefore they no longer appear in the Customize menu.
Thus there is now only one menu for showing/hiding columns and that is the new ‘Show fields’ or ‘Hide’ button (depending on the A/B test you are currently part of).