Although I don't want to entirely rule out that there could be a few sites also in Denmark that are so inaccessible (like on a distant uninhabited island) that the backcountry-tag could be considered, I generally don't think the tag makes any sense here. The way I understand the OSM-meaning of backcountry (also from the discussion) is that it is a place that is so remote that you have to walk for several hours to reach it. Not a place you can simply drive up to in your regular car and walk a few minutes to reach. I have yet to find a campsite in Denmark that is actually more than a few kilometers from the nearest road (that is accessible by car). Regarding needing more information than the camp_site-tag itself contains: You can still add the additional features tags to tourism=camp_site which includes  drinking water, toilets, etc. and of course you can also still map features such as fire pits in the area and tag those with wood_provided=*