You may or may not need backup heat, it depends on your particular home. Heat pumps DO still capture heat below freezing and down to temperatures much, much lower than they used to. Mitsubishi, for example, sells air source heat pumps that maintain 100% heating capacity down to 5 dF and can still heat (albeit at reduced capacity) down to -13 dF: https://www.mitsubishicomfort.com/articles/keep-warm-this-wi...

We model decreasing heat pump capacity and efficiency as the outside air temperature declines. Yes, heat pumps do lose capacity and efficiency the colder it gets, but they can still provide heat even when it's very cold out (there's heat energy in the air until absolute zero, or -273, after all!). You can see that declining efficiency if you expand the "Step 2: Choosing your ideal heat pump" section. We'll add a graph of capacity relative to outside air temperature as well.

Backup heat is still often necessary for many homes in cold climates, but it's necessary a lot less often than you might think. Note that in the results you'll see for the site, backup heat is included if necessary for single-zone centrally ducted heat pumps, but not for any other ducting types.