Biologically, cats are known as “obligate carnivores”, meaning they are biologically obligated to eat animals to be healthy. Their teeth aren't designed for chewing tiny, dry meat crackers. They're made for catching and ripping live prey!
Although cats may be able to survive for a number of years on a diet low on, or void of, animal meat, there have been many recorded cases of cats becoming ill, losing their eyesight, and even dying when they aren’t fed meat.
Cats (big and small) require zero carbohydrates.
No grains, fruits, vegetables.
We can give cats tons of plant-based protein but unlike dogs (and obviously humans), they aren’t set up to assimilate it properly so they may become massively deficient without animal meat. Whether it’s raw food (usually most biologically appropriate for cats), canned food (second best), or dry food (not recommended but better if carbs are absent), the research still isn’t conclusive on whether or not cats can truly be healthy on a vegetarian diet.
Better safe than sorry in my opinion.
PS Cats need to get almost all of their water from their food (raw animal flesh contains a large amount of water), simply because of their biology. So if you can't or won't feed them raw food or canned food or a combination of both, soak their kibble in water for 10 minutes before feeding. It's not ideal but it's a hack that is better than nothing.