There do exist brands of “Vegan Cat Food” that have protein supplements that the manufacturers claim will meet the basic needs of the animal.

that the manufacturers claim will meet the basic needs

https://www.bluecross.org.uk/advice/cat/food-and-weight/can-cats-be-vegan

Cats have specific nutritional needs, including protein requirements and amino acids (such as taurine and arachidonic acid). These needs cannot be met by a vegan diet without synthetic supplements. Additionally, taurine, (an amino acid, which is needed for many of your cat’s vital tissues and organs including their heart, eyesight, and immune system) is an essential part of a cat’s diet. Cats cannot make their own taurine so they must have it in their diet. Natural taurine can only be found in animal-based proteins.

where is the science, the studies, etc. that prove the safety and benefits of vegan alt food (both short and long term studies that aren't funded by said manufacturer)? Nowhere? okay then.