Not sure how useful this would be, but I have menu items points to tags that haven’t been assigned to any posts yet. Publii is aware that there is not a page generated for this, and displays the cross out. The user has the option to manually hide/unhide this menu item.
I think a nice feature would be to have Auto/Hide/Show states, where the default behavior is to hide any menu items that will point to known missing pages.
Since Publii already shows us that a menu item is pointing to a missing item, Publii should also be able to tell us, for menu items that are in the Auto state, whether they will be automatically shown or hidden.
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