Eating only Meat, Egg, Milk, etc and of course your body needs water although on provided by animals. I’ve heard people doing it and they’ve been very healthy and have even lost lots of weight and have gotten very strong.
It’s not optimal. There are no studies showing the effects of this diet long-term (even the Inuit who arguably have the least access to plant foods have traditionally eaten some plants, and they are also genetically adapted to not be in ketosis while eating a mostly fat- and protein-based diet).
I think the reason it works for some people is that they are eliminating most processed foods, processed carbs, and potentially some allergens that they specifically are sensitive to. Also, if you are eating a pretty monotonous diet, you will generally diminish your caloric intake.
Technically, I suppose you can survive, but there is no reason for you to do this, especially considering the fact that the vast majority of scientists who study nutrition will strongly advise against it.
Edit: spelling
You could depending on the fat content of the meat, milk, cheese, etc. It’s not optimal, though. Fruits, greens, beans, etc provide fiber and other micro nutrients that have legitimate health benefits.
We really don’t have much long term evidence on this type of eating pattern, but
The Masai did have atherosclerosis: https://academic.oup.com/aje/article-abstract/95/1/26/167903
The Inuits were not in Ketosis, due to a a special gene: https://pubmed.ncbi.nlm.nih.gov/32088118/
Another nutrient of concern is Boron. Boron is needed to lower your SHBG, which blocks testosterone. The only animal source of boron is testicles, which ancient athletes ate to boost their free testosterone. However, it is much easier to get boron from plants, as it is a fairly ubiquitous nutrient in fruits and legumes, especially raisins.
I’ve been carnivore for 3 years and will unlikely change back. I was clean keto for 5 years before that. Fat is important, don’t skimp on fat, it is your main energy source. Also make sure to get your electrolytes, especially early in the transition. Head to the carnivore or zerocarb subreddits for more info and faqs.