Old heat pumps didn't do well in old temperatures. In the 80s and 90s, a heat pump couldn't handle freezing temps.
Modern heat pumps do fine well below 0 degrees Celsius. Here's one that's great to -15 C and okay down to -25 C[0]. If you search cold-climate heat pumps, you'll find plenty of information about how modern heat pumps are fine in most the temperatures you'll find in most of the US (including up north).
(I have heard that a lot of them are still only available in Europe, but you can definitely find some in the US.)
Modern heat pumps do fine well below 0 degrees Celsius. Here's one that's great to -15 C and okay down to -25 C[0]. If you search cold-climate heat pumps, you'll find plenty of information about how modern heat pumps are fine in most the temperatures you'll find in most of the US (including up north).
(I have heard that a lot of them are still only available in Europe, but you can definitely find some in the US.)
[0] https://carbonswitch.com/best-cold-climate-heat-pump/