I’ve had ok results doing a week of nearly vegan at roughly 500 calories per day eating a lot of greens and other low carb veggies with plenty of salt. Works out about the same as 4 days of fasting for me. Both approaches are similarly challenging for me so I do the veg thing when the garden is pumping out produce.
I expect Frankobear intends something higher calorie for longer duration, perhaps months. In my case I wouldn’t be too concerned about lowered metabolism but rather about diminished results of weight training for an extended time without animal proteins. I expect those and perhaps other issues will vary a lot person to person.