It would be useful for the Workshop browser to be able to distinguish an “Unlisted” item from the rest. “Unlisted” would not appear on the ingame Workshop Browser but would appear in your Favourites; the Workshop items would still download and display as per usual ingame, they’d just be missing from the Workshop Browser.
Unlisted Workshop items could still appear to Admin in the official Steam Workshop and as such would still be possible to moderate.
In the case of Playermodels:
If you encounter a Player using an Unlisted model, you can still highlight them in the Tab menu and Favourite the model from there for future use.