A person will starve to death if they remove all saturated animal fats from their diet, not to mention vegans will die from pernicious anemia after their stored hepatic B-12 supply has been consumed.
https://en.wikipedia.org/wiki/Protein_poisoning
B 12 I’ll give ya vegans and such should supplement it, but protein can be made in your body from and/or is found in greens and beans and some grains, nuts, seeds, etc. Animal protein unless it’s clean fed/raised, can cause some people some problems.