i just did a search for camp sites in Jersey , of the four listed two have 'site closed ',in the details, would it not be better to delete these sites from the lists.
The owners may have ceased trading as a campsite but could be waiting for a potential buyer in which case they may be inclined to want to look at the past reviews to see what the general feedback is and what improvements can be made. It may not be the case but just a possibility that's all.
If the site is shown on an OS map, its also useful to be able to look it up, even if closed, rather than turn up and find its closed, particularly if you are touring and not booking sites in advance.