Earlier quoted context omitted.
There's nothing inherently unhealthy about being in a temporary state of negative energy flux (unless you are already underweight or deficient in an essential mineral or something)
This will very quickly devolve into a definition of an unhealthy diet, or healthy vs unhealthy in general, none of which is well defined. From the perspective I took where a healthy diet is a sustainable one with sufficient nutrients and long lifetime expectancy, a weight loss diet is unhealthy and unsustainable, but there to negate a different issue and improve long-term health.
However, a diet that varies in how 'fed' you are over time is very likely healthier than one where you are constantly taking in nutrients. During that post-prandial state, insulin and growth factors are elevated, and autophagy and other important pathways for 'tidying up' sick cells are turned off. That is why I don't think it is fair to say that fasts or restrictive diets are inherently unhealthy, I reckon 'whether this can be sustained steadily forever' should not be the bar. Similarly, the 200% of normal diet consumed after marathon is healthy that day and probably more healthy than eating normally since it is going to allow for repair and replenishment of depleted substrates, but would not be healthy if eaten every day.
For someone with health issues stemming from obesity, a potato-crash diet could immediately improve health, and continue to for a good while.