kant himself was not a vegetarian, and did not advocate for it. modern philosophers who have attempted to shoehorn animal rights into Kantian ethics are thoroughly rebutted.
I didn’t say anything about Kant himself (Kant also thought that non-human animals were basically just “things” without rationality or self-consciousness, which is however in direct conflict with the current scientific consensus. Kant still argued in favour of treating animals “humanely”, just not for their own sake). Anyway, some well-known and well-respected contemporary philosophers who argue(d) from a Kantian perspective in favour of animal rights include e.g. Christine Korsgaard or Tom Regan, and many lesser known philosophers (see e.g. here for a recent example). I also see no indication that these types of arguments as a whole are supposedly “thoroughly rebutted” (not that serious philosophy really works like that anyway). Some other philosophers disagree with some of their arguments, of course (this is normal in philosophy), and many don’t subscribe to Kantianism in the first place, but afaik most of them tend to take issue with how Kantian ethics is applied (or that it is applied) moreso than that they’re trying to defend animal exploitation as such. Either way, none of that changes the fact that ethicists have been using Kantian ethics (among many other meta-ethical frameworks, as I said before) to argue in favour of animal rights, and that there aren’t really many arguments in defense of killing animals for food (in particular in the context of factory farming) that find widespread support (among moral philosophers, that is).
I knew youd reference korsgaard. but her view is far from mainstream and incisively dismantled in Why Kant Animals Have Rights?
The author of the article you mentioned (Alex Howe) is a good example for what I insinuated: He takes issue with Korsgaard’s argument (and, it seems, Kantian ethics more generally), but in his PhD thesis argues for granting “basic citizenship rights” to domestic animals (including farm animals), which is arguably a far more radical position than veganism, which merely posits that it is immoral to exploit (or be cruel towards) animals (e.g. as a food resource). Either way, if you have an issue with Kantian ethics and how they are applied to animal rights, I suggest you take it up with a Kantian (which I am not). And, even if I am repeating myself, none of this has any direct relevance to my earlier point, which is that many moral philosophers from many different schools of thought (including, but by no means limited to, Kantian ethics) have arrived at conclusions which are at least similar to the basic vegan stance, i.e. that unnecessarily causing harm to (sentient) animals, e.g. by exploiting them as food, is immoral.
