SlimFast and caffeine pills were a terrible idea, but in the 90s that was one super popular way to lose weight. The Strawberry, Chocolate, and Vanilla shakes didn't taste awful and were intended to replace one or two meals a day. This crap taught me no new behaviors around eating, and didn't help me understand anything about my health except calories in need to be less than calories out. The weight fell away, however, until I got exhausted and went back to eating normally.
This video shares the history in detail: