I think there are just too many variables to make any blanket rule. Really it comes down to being a sensible pet owner who takes the time to really care. Not all outside bunnies are "outside" ie just in a hutch - mine are outside but their hutch is a summer house that is lined with relective plasterboard. I haven't heated it at all, though it has always been an option, because they just haven't needed it. Despite outside temps of down to -10, their water bowls have never frozen, so there is a very clear distinction between good unheated shelter and poor.
Access to movement is a big one as well - all animals can create warmth by moving, and those bunnies who have plenty of space and reason to explore it regularly ie a variety of feeding stations, hiding treats, food toys that move will stay warmer than those who have to stay in one spot in order to eat. We know in horses that they are often warmer outside with shelter, than in a stable, because of movement and being surrounded by cold walls and floor.
Flooring is a huge one, and probably overlooked - anyone who has camped in the cold will know that if you can block the cold from beneath you have a much better chance of staying warm than piling blankets on top. Hutches on stands are probably warmer than hutches on the ground, but many hutches will be placed on concrete I suspect, and just lifting thses and placing onto thick rubber matting could make a huge difference.
But at the end of the day it all comes down to people using common sense, observation, and meeting their individual animal's needs. Acknowledging that bunnies are warmer with a friend. Watching their weight closely as an indicator of energy requirements. And if you can't keep the water bowls from freezing, then I'd say the bunny is likely to be too cold too.
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