bavmouse
Sidebar content - icons - are simply bookmarks to certain locations in the filesystem (folders). User may add and delete sidebar icons as he wants.
If a location gets removed from the filesystem, its icon in sidebar is not removed automatically. A user should remove it by hand.
Another thing to remember is that Nautilus doesn't allow to remove from sidebar the icons which point to XDG-defined folders.
On your screenshot XDG-defined locations and their corresponding sidebar entries have symbolic icons, which reflect XDG-folder content. These are Bilder, Dokumente, Downloads, Musik, Papierkorb near the top of the sidebar on your screenshot.
Five old sidebar entries are at the bottom of the sidebar. They have the same names, but have the generic folder icon. These entries do not work anymore. They should be simply removed by hand from the sidebar.
Remember - if you try to remove a user-defined or broken sidebar entry (icon), then Nautilus does allow to do so. But if you try to remove XDG-defined entry, then Nautilus does not allow to do so. In other words, there's no space for human error here. There's no way to remove a "good" sidebar icon. Only "bad" or user-added icons may be removed.
Go ahead and remove 5 bottom sidebar entries having the generic folder icon.