Working with this feature now — a small request would be to allow topics to be selected as part of custom categories (instead of just links). Splitting the “Categories” up into multiple sections would be nice.
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
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.