I think it depends not only on the breed of chicken and their overall health, but on the owner's tolerance. Yes, the owner. There are chicken keepers on this site who use no supplemental heat at all and let the chickens range at 10-20-deg below (F or C, I don't know). *I*, on the other hand, can't sleep thinking of my poor shivering birds out there in anything below 10-deg-F. I value my sleep and health, too - so I compromise.
The only time I close the pop-door
and provide supplemental heat is when there is a nasty snow or ice storm going on, winds are blowing hard, and temps are brutal (IMO like below 10-F). Otherwise, I let the chickens choose where they want to be - in or out. If there's no precipitation, but temperature is below 20-F, I'll leave the pop-door open but also leave a heat lamp on in the coop. This way they can escape the cold if they need to. Mostly they stay outside, but they'll go in and out during the day. I keep food and water outside (except for when I have to lock them inside during storms and bitter cold).
I do have a different kind of setup, though. My coop is an old camper trailer, up on blocks. So there's about 12-16" of headroom underneath, with all dirt floor. Over time, the birds have dug holes, or depressions, in the dirt - so it rather looks like a mine field under there. Chickens will snuggle down into the holes, and if I look underneath during a storm or very windy day, often I'll just see their heads poking up. Rather comical!