The *vast* majority of pheasants in North American aviculture, barring the polyplectron, argus, junglefowl, and a few others, are incredibly winter hearty.
I live in western Canada, and we routinely get temperatures of -30 Celsius for weeks at a time around here. One of Canada‘s largest private exotic pheasant breeders lives about 15 to 20 minutes away from me, and she keeps all of her birds outdoors, totally unheated, year-round, with their only shelter being a little three sided lean-to, but the majority of the birds don’t use them, they either just sleep on perches, or hunker down in the snow. She maintains 15 different species. I’ve been to her place before and seen her set up, and her birds are all in immaculate condition. Strong, healthy, and beautiful plumage! They also breed prolifically for her.
Personally, I’ve kept Golden, Amherst, Mikado, Humes, Temmincks Tragopan, Blue Eared, Impeyan, and Silver pheasants, and they were all housed outdoors without any supplemental heat or shelter, and did totally fine, even during the winter. However on the same note, I’ve also had Argus, three different peacock pheasant species, and Edwards pheasants, and they all definitely require supplemental heat to varying degrees in the winter.
Generally speaking, as long as the birds are able to get out of the wind and completely cover up their feet while perched, and have a good quality diet, they can stand absolutely miserable weather with ease.
As a general rule, in most public zoos, you would find the birds have indoor access during extreme temperatures, but that’s not necessarily because birds require it, per say, and usually more so just because of facility preference/requirements.