The reason cats can't be vegan is that they cannot produce an amino acid called taurine, which is something dogs and humans can produce (but which we also get sometimes from dietary sources).
Most dietary sources of taurine are meat. This is why dogs and humans "can be vegan" but cats "can't". However, vegan taurine is made and can be bought as a supplement, both for humans (if you want to ensure you get some taurine in your diet), but also in properly made vegan cat food.
It seems to me then that cats can be vegan, just not without intentional effort to ensure proper supplementation of taurine. That is, they couldn't be vegan in the wild (where the only source of taurine is meat) and you can't just start to feed them a vegan diet without taurine and expect the cat to be healthy and survive.
In fact, cats fed a proper vegan diet tend to have better health:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10499249/
I think the question is really what you are feeding your "vegan" cat: if you have managed to find (or make) a properly fortified vegan cat food it is theoretically possible to feed your cat a vegan diet.
This all feels a bit like the "controversy" around feeding young children and babies a vegan diet: done poorly it can be catastrophic (pun not intended), but it's entirely possible to have a healthy vegan diet when enough effort is put into ensuring nutritional needs are actually satisfied.
That said, I also know of two other vegan responses:
In fact, cats fed a proper vegan diet tend to have better health: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10499249/
It actually never definitively says that in any of the studies mentioned... This particular study relies entirely on self reported results, with less than 10% of the sample sizes being fed a vegan diet, with no actual controls in place. It's a meaningless study. It honestly reads like a fluff piece where they collected some surveys from an already pro-vegan community. As we've seen from the rhetoric surrounding this situation some vegans will absolutely feed their pets inadequate food and feel good about themselves while doing it.
And the final nail in the coffin:
This research and its publication open access was funded by food awareness organisation ProVeg International (https://proveg.com).
Ahhh... there it is.
This research and its publication open access was funded by food awareness organisation ProVeg International (https://proveg.com).
FUCKING LMAO
Funny how you don't even mention what that is...
This research and its publication open access was funded by food awareness organisation ProVeg International (https://proveg.com). AK received this award ID: Oct2019- 0000000286. However, this funder played no role in study conceptualisation, design, data collection and analysis, preparation of the resultant manuscript nor decisions relating to publication. We are grateful for their financial support.
They would have to be total fools to write anything other than that; they're not going to admit their research has a conflict of interest. Their statement that their funding source didn't affect their research outcomes is worth about as much as a pinkie promise.