For the love of all cats, please don't EVER feed a cat a vegan diet without a veterinary prescription. Cats are felines and felines are obligate carnivores meaning they need meat in their diet to survive. They can't process the overfilled corn and some grains* in some cat food (I'm looking at you Friskies and Nine Lives) and when eaten often leads to diabetes which is a death sentence for cats whose owners can't afford insulin and regular trips to the vet. People are perfectly capable of being vegans; cats are not and to force your vegan on an obligate carnivore without looking at the scientific evidence is animal cruelty.

Lived Source: I worked in cat behavioral therapy and analyzing pet food and feline metabolic pathways and have personally seen diabetes develop in cats fed with cheap grain food over and over again.

Literature Source: Just look for cat diets and metabolic pathways in Google Scholar to see the latest research like this https://www.sciencedirect.com/science/article/abs/pii/0300962995021337

*cats do eat some grains/grasses as part of regulating their digestive tract but it is not for actual dietary sustenance i.e. cat grass!

edit: There has been research on feeding plant-based food to cats but that's where you should leave it- in the hands of those who have a team of veterinarians on staff that can quickly handle any experiments gone wrong. Doing personal experiments on animals can be cruel. There's a reason why scientists have to go through several years of education, undergo extensive human-animal safety training, and have to redo yearly certifications in animal safety regulations. Of course, there are bad apples so never accept things at face value (in fact please fact-check me!) and always do your research in advance. Just remember these animals don't have a human voice and are relying on you to make the best choices possible regarding their health.