When I researched this year on replacing my A/C / Propane Furnace system with a heat pump, I found that companies didn't seem to want to advertise what temperatures their heat pumps can operate effectively at. If I look at some marketing materials from Google it seems companies like Carrier and Trane are only willing to talk about their heat pumps working in low temperatures if it's regarding their top-of-the-line (very expensive) variable speed compressor units. No one talks about what temps the mid range units can handle, and I'm guessing it's because they don't work well below 20F.
I am just saying what the typical install is in the South East US. The heat pumps installed may produce some heat at 5F but they can't keep the temperature to the set value, so there are resistive elements (Aux heat) to make up the shortfall.
It's pretty common for people with heat pumps to have Aux Heat kick in during cold spells, which cause power grid overload issues.
I realize you can insulate a house well enough and have a good enough heat pump to avoid backup heat, but 5F or 0F days are rare enough that the codes do not enforce this.
My heat pump is advertised to operate as low as -13F. One thing to note is the efficiency of heat pumps is not optimal when they are operating close to their extremes.