For me this is not an edge case.
A forum (i think hosted by discourse) I visit used the legacy menu untill yesterday. Today the new menu is shown. But since no one added any defaults, it looks like this
Users miss the opition to navigate to /categories or choose a categoy like they could with the hamburger menu. It is quite difficult to notice that there is a new tab in your settings, in which you can configure this menu yourself.
If the Categories and tags sections would show there would be an option in the menu to navigate to /categories and /tags. Also the pencil icon would show, so it would be easier to explore the new settings.