Not a HVAC specialist by any means, but just did a quick google on which are the most efficient heat pump systems available today.
Found this link that mentions there are Mitsubishi units that can maintain a COP of 1.8 down to -18°F (-28°C), which is likely enough to not need any supplemental heat at all when correctly sized.
Do Heat Pumps Work In Cold Climates? | Building Advisor
Mitsubishi's marketing page doesn't say much how it works, just that it maintains 100% heating capacity down to 23°F and 76% capacity down to -13°F, making supplemental heat unnecessary.
Year-Round Comfort, Even Below Zero | Mitsubishi Electric
Googled for how it works:
"Mitsubishi's hyper-heating H2i® compressor motor signals that a burst of heat is needed when the temperature gets below 40°F. Depending on the temperature, the motor revs up accordingly and the system absorbs the additional heat thrown off the compressor magnets. After the extra heat is absorbed, it's funneled to the back of the blower."
That sounds very much like what Tesla describes about using the compressor as a heater.
Hyper Heat Feature Review
Fujitsu's system claims it works down to -15°F but it seems they just do that by having a separate heating system for the base unit that keeps the condensation from freezing (without having to run the unit in AC mode like most heat pumps do to defrost).
XLTH Low Temp Heating: Halcyon™ MINI-SPLIT TECHNOLOGY - RESIDENTIAL - FUJITSU GENERAL United States & Canada
From same blog above:
"Fujitsu's Halcyon™ XLTH Low Temp Heating technology works a little differently. It uses a metal fan guard, a base heater and base drainage holes. The metal fan guard is more capable than plastic guards other heat pumps use to withstand colder temperatures. Including a base heater means when the temperature gets below 36°F, the built-in heater warms the base of the outdoor compressor. Warming the base allows the compressor to capture the warmers' heat, and doesn't allow condensation to freeze - which can damage your compressor. And the drainage holes let melted water (from snow and ice) to drain out of the unit."
So not technically supplemental heat (as it's not warming the air), but it's still using a separate heating system (although it's to heat the equipment).