There’s no option to Enable custom sidebar sections in the settings menu
After adding a custom sidebar section using the + option at the bottom of the sidebar, on needs to click on the Edit icon on the sidebar which edits the custom sidebar selection directly. If the custom sidebar section is created by the Admin then it applies to everyone (unless I’m missing something). Users can however create more custom sidebar sections if they choose but cannot edit the ones created by the Admin
As RBoy says, I think this is no longer a setting now and you should have the buttons to add and edit the sections ready and waiting on the navigation menu.
Also worth noting that “sidebar” has been deprecated in favor of “navigation menu” in the core and in settings documentation and most labels. Searching in Admin-Settings for sidebar is likely going to yield incomplete results.
I added a link for preferences /my/preferences but it is also visible to users who are not logged in. Is there any way to make it visible to only logged-in users?