Okay it's almost 2023, I think it's time we talked about the "vegans" who purposely inflict suffering onto animals by feeding them incorrect diets or not doing correct medical care. I think any good vegan who geniuely believes in the betterment of animals would NEVER purposely go out of their way to force feed incompatible pets vegan diets that can later cause suffering, illness and death. It is only ever done for their own personal ego and gratification and I personally see that as worse than using by-products. You're actively inflicting harm to cats, dogs and lizards when more suitable pets like rabbits, guinea pigs and hamsters are RIGHT THERE. Who are already compatible with your belief system.
I own a cat who is under my care until he passes. I will ensure as he is a care dependent that I am maintaining his welfare and health by feeding him a meat diet even if I personally am vegan. He can not change his diet and I have owned him for over a decade so he will not be punished for my changes in life. After he is gone, I will likely rescue more rabbits (I have had 3 so far) as they fit my lifestyle and beliefs much more. But I will NOT enforce suffering on my cat just because his existence conflicts with my personal diet. I just won't own another cat after he is gone.
To me that is the true Vegan way. If you can go for alternatives in food then you can pick a different pet. If you are already committed to the care of an animal who needs to eat meat then it is more vegan to ensure the optimal health, wellbeing and happiness for the animal. Not to cause unnecessary harm and stress to an animal just because YOU personally do not like something. If you really find it distressing then rehome the animal to owners who will put their wellbeing and life above all else.