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.
i think the admins at that forum have not yet set up the default sidebar categories and default sidebar tags in admin settings. then the users should be able to configure their own sidebar categories in their preferences side bar section.
As a user you can also change your settings before the admin chooses a default. But therefore you have to notice that there are new settings. Since you don’t get a notification like “there was a software update, the design of the menu was changed, you can personalize the menu in your settings.”
So everyone thought it was a discourse bug that almost all links disappeared from the menu.
That’s why I think a default which shows the options to personalize the sections is a better default than an empty menu.