I track and comment on a CICO site since ages and it’s very very very basic info there that we shouldn’t eat too little calories. I believe we should keep our deficit not too big (what is too big is quite individual and our fat reserves matters a lot) but the experts there says we must stay at least a bit above BMR, that’s stricter.
So even saner people focusing only on calories don’t just starve.
It’s very known it does bad things to our metabolism and not only that.
@Sivart1966, it was quite horrible to read your comment. Stop it. You need to eat way more… I know I need a calorie deficit so I try to eat below 2000 kcal (I am a slightly active overweight 45 years old short woman). But I can’t function below about 1600 and even if I could and wouldn’t feel hunger ever, I wouldn’t go too low (like 1200 as so many woman thinking a female must go below to lose fat. how ignorant is that? it’s super low - but some tiny women may need that. no one needs 600, that’s almost nothing, not even slight starvation than 1200 would be to me).
There is no such thing as proper ratio. If you eat “proper ratios” under 600 kcal, it means you barely eat any protein… You lose muscles due to starvation already but if your protein is super low as well, it’s quicker. Your body need amino acids and your need is in grams. I eat about 600 kcal protein alone, sometimes less, sometimes more. No, it doesn’t even put me out of ketosis and surely don’t make me stall. I always eat high protein, even on my low-cal days, even when I lost fat just fine, it was high. Not crazy high, I wouldn’t like to eat, like, 300g protein but my need is smaller than bigger, more muscular, more active people. Even 300g may be quite okay regularly for someone. But 600 kcal for everything, that can’t give you even remotely good macros. Forget about percentages, grams are way more important. Of course, you may find you like some percentages better but your keto won’t be good because your percentages follow some random fixed idea. Obviously different people need different percentages. Like it’s best if my carbs percentage is 1% and my fat stays below 75%, more like 70% if I want to eat little enough. Would I need 3000 kcal? I would raise my fat percentage as I don’t need more protein (I already eat high protein but it’s the best, easiest, most natural to me) but I gladly eat fattier food. Some people works better with 80% fat - but they need a high enough calorie intake to get enough protein at the same time…
Net carbs are never negative. I repeat things from others but maybe it sinks in better? 
There is no such thing as stalling a bit. Your weight is the same or not… Do you mean your weight-loss slowed down? Even if it stops for weeks, it’s not a stall - though at 600 kcal it’s worrying. But again, anything is worrying at 600 kcal as it’s very much inadequate nutrition. Oh yep, calories are one thing. Even if you get a ton of calories from your big fat reserves (I have only 40lbs extra at most, I probably can’t keep a big deficit but very heavy ones have a chance), you still need various nutrients and it’s hard to get from so little food. Even the minimum protein and fat intake is hardly fits…
I don’t know. Maybe because many people like fasting and they can do it better on keto…
I like fasting (mostly IF) but it gets harder when I eat less carbs 
Some people are natural fasters. Many of us can do at least IF on high-carb just fine, automatically, without even knowing it has a name.
And some people are bad at it, no matter their woe. And there are everything in-between.
Fasting has benefits but I never would advice it if it causes problems. Hardships - it depends. If I get hungry (or just not satiated so well), I eat and I find this right, I have no problem with being a tad pushier but if fasting cause suffering, it’s probably a bad idea.
Do you have some idea about your protein need? Whatever it is, it’s very unlikely 100-150g protein would cause problems and it makes zero sense that it would stall your fat-loss, no matter how I look at it. Especially on keto.
Oh and you need to eat, hungry or not. I am rarely hungry nowadays, it’s good I still feel I need to eat. But if you feel nothing, you still shouldn’t starve. And yep, fasting is better than starving below 600 kcal, it has a different effect on your body - though overdoing it is bad for similar reasons.
I think it was mentioned because you wrote you aren’t hungry. Don’t eat then (for a little while) but don’t do this starvation diet. BUT if you have some eating disorders, maybe not even that is a good idea… And you should be able to eat properly when you stop your fast (maybe not immediately and the longer you fast the more careful you need to be with refeeding) and shouldn’t start a longish fast again immediately. You need your nutrients… A few days fast here and there should be okay if your body can handle it (mine complains, slim people don’t have the reserves to gain energy… it’s not for everyone) but eventually you should eat.
And if you ate very little for a while, I would think undoing the problems (metabolism slowing happens quite quickly as far as I know. but it’s easier to make it okay again after a short term starvation) is more important than doing some fasting. But if you really can’t eat enough sometimes, it may be an option occasionally.