Some metabolism loss is real (some serious science in the articles linked in my prior post from earlier today), but "starvation mode" is not. You can drop your daily caloric requirements by as much as 400, but that is very rare. Dropping by 200 is not that rare if you lose a lot of weight and the bad news is that you don't bounce right back. Anyway, that means that very few people can maintain their weight on under 1200 calories a day. There is only so much efficiency gain available; keeping a body above room temperature, pumping blood, respirating air, digesting food, walking, etc are physical processes that require energy. If you approached it as a physics problem, the amount of work and the energy to perform it would be less than we usually require because we are inefficient. The body does become more efficient when calories are reduced but there are limits.

What really sucks is that it does not reset easily. Someone who weighs 200 and can maintain it with 2300 a day might lose to 170 expecting to maintain it at 2000 and find that it is actually 1800 because of the metabolic adjustment. Then they gain back to 200 and now it only takes 2100 to maintain, so if they return to their old eating habits of 2300, they gain even more.

That's the bad news. The good news is that it does seem to stabilize, so if you can get used to eating the lower than expected maintenance calories, you can maintain the lower weight. Very few people do; studies I am reading have success pegged at about 8% of people who maintain a substantially lower weight long term. One study referenced from one of the articles I linked found that most of the people who do maintain are still diligently tracking their weight and diet.