Modern air source heatpumps, those that are actually designed for heating around or below freezing point, for some last 10 years, have improved defrosting quite a bit.
It's mostly a software thing.
From physics perspective, defrosting is not a problem; when condensing outdoor air from gaseous phase to liquid then ice, the latent energy comes in our advantage. When defrosting back to liquid (+ a bit of steam), we just lose that part of energy we already gained. From physics perspective, the result is the same as running continuously in a perfectly dry air with no defrosting need.
In practice, there are losses in the defrost process but that subjects it to optimization. If you run 5 minutes of defrosting every 30 minutes "just in case", it's going to be a huge loss of COP. First, it's important to reliably detect the need of defrosting. This is not too difficult, look at the difference between the evaporator coil temperature and outdoor air temperature for example, if layer of ice is blocking the airflow, this difference increases. The key is to place the sensors appropriately, or even use multiple sensors, test the shit out of the system in different conditions and finetune the algorithm.
Then the defrosting itself, it's best to use as high power as possible to do it quickly, but then it's important to stop the cycle as soon as all the ice is gone. Any excess time spent heating the outdoor air is waste.
The Big Names, mostly Japanese but also some Korean and Chinese, have managed to solve this problem pretty well. There are occasional issues like the well-known Mitsubishi Electric suddenly delivering a new series of "high-quality" "Nordic optimized" heat pumps where significant percentage has completely broken defrosting logic, but such failures happen on any field time to time.
But in reality, in modern machines, the COP drops like 0.2-0.3 units in frosting conditions, but no more than that.
And be aware that the need for defrosting begins already above the freezing point, around 3-5 degC, because the thing cools the outside air so the evaporator coil runs somewhat cooler.
Very cold air cannot hold much moisture so the amount of defrosting is reduced in very cold weather.