Fasting and starvation tactics are bad because they are essentially depriving your body of what it needs to grow. So your body is basically digesting its self to run its basic needs. It digests fats, but at the same time it also starts to digest muscle and other tissues.
A laxative diet is the ingesting of minerals or chemicals that help you defecate (such as magnesium) which can be harmful in several ways. They can prevent your body from properly absorbing all of the nutrients in the food that you are eating and can disguise other issues such as diarrhea, which in some cases can be a symptom of something serious. (And also has the potential of giving you a hernia or irritations.)
Liquid diets are diets that consist of liquids. which are often times missing a large chunk of basic necessities of your body, from some missing fiber or iron. To some just being plain ignorant. There are a very few that actually have everything you need to grow and survive.
