This is not something you want happening in a well-nourished individual. Beyond making you more likely to die to predation or accidents from severe muscle wasting, it also just feels horrible. There's a reason why people with untreated hypothyroidism (unrelated to starvation) struggle with exercise and weight loss.
I've also personally observed some people on... inadvisable extreme crash diets getting some weird bloodwork numbers. Like TSH spiking by a factor of 10- which, unlike the above starvation case which typically suppresses TSH, may imply malnutrition and inability to produce sufficient thyroid hormone. Their empirically derived caloric burn rate dropped by more than 30% over the duration of the diet, and a substantial amount of that was from dramatic muscle wasting. Not exactly ideal!
So the body has definitely way to sense and adjust in some areas.
As your body sheds fat, it no longer needs to expend energy building, preserving, and keeping your fatty tissue warm. JIT energy use instead of lossy battery storage. It doesn't further slow your metabolism beyond what you are genetically predisposed towards, but metabolism does slow (a little bit). Another way to think of this is, when you consume in excess of what you need for healthy living, your body begins consuming additional calories in order to build and support the additional long-term reserves you are creating and maintaining. We have decades of research supporting this notion (as well as common sense).
There may also be an aspect of "hibernation" that kicks in when dramatic caloric restriction is suddenly introduced. That is, you are further inclined to be less active as you lose weight quickly. I'm less certain of that, though.