This treatment was widely used to control epilepsy before the advent of effective seizure drugs. The extreme difficulty of maintaining this diet made drugs the preferred choice. It is still used for epilepsy that resists drug treatment and doesn't really fall under quackery.

I wonder how many mainstream approaches to health will be considered quacky in 15 years.

Any examples in either direction?

Here's a fun one:

http://idlewords.com/2010/03/scott_and_scurvy.htm

I'd guess the percentage of approaches considered "quacky" which are later proven to work, is fairly constant over the ages. I'd also guess 80/20.