What's the moral problem with cannibalism? I can see the taboo originating because of health concern and the contamination from the assumption that the cannibal kills an innocent to eat him (and the taboo against desecrating a corpse), but by itself, is there something morally wrong to eat sentient flesh? Using body parts of the deceased carries no moral value in itself: it's socially accepted with organ transplant. We don't see it as an evil necessity to save a life but as a thing that is morally neutral. I am pretty sure that if there were another sentient species, we'd readily eat them, much like we eat pork, whale, squids, dogs and monkeys, unless we had granted them the same rights as humans ; and then we wouldn't eat them not because it's morally wrong but because body part would presumably be as difficult to find as human ones are in the real world. Much like very few countries have actual laws against eating legally acquired human flesh ; it's just that it's impossible to find the required product. There is, for example, no legal problem with making human cheese (
Breast Milk... Cheese? You've Got Questions, We've Got Answers). It's absolutely gross to me, but I think it results from the social taboo, not any moral explanation I can imagine against it, not different from the taboo I have against eating insects (which are a delicacy in many parts of the world).
So while your stance on cannibalism is that it's a moral wrong to eat sentient species and it was called man-eating because there was no other sentient species, my stance is that the taboo originated specifically from eating
Homo sapiens after killing/desecrating it, not as a general taboo against eating sentient beings. It also depends on how alien you make your sentient species. If they are humans with pointy ears and darkvision and you consider orcs, humans and elves as six sexes of the same species, I can see a general taboo against sentient-eating emerging, as the taboo against "eating self" would apply. On the other hand, if you have a perfectly sentient species evolving with say, praying mantises like the thri-kreen, and praying mantis practice sexual cannibalism, would they evolve a taboo against eating humans?
In a fantasy world, I guess it would be a strongly contested practice and induce horror and be used as an excuse to go to war (these humans are hunting elfs like us! they say we taste like chicken!) and even within human communities the debate would be strong, but I don't see a published setting adressing it (they are eschewing a lot of things like gender inequalities, racism and slavery, I don't see them trying to include dietary habits of sentient species in a poly sentient species world ; especially if it's to have "it's OK to eat elves, but dwarf eating is gross!" and I can't really see it coming up a lot in a typical game anyway...). I'd also see a taboo against destroying corpses being much more prevalent, since it can block Raise Dead, so cremation would be seen as an evil thing to do to an opponent, even more than killing him, since you kill him and make his return more difficult.
The OP asked about an "inherently evil" biological species, the illithid came to mind, but I think their inherent evilness comes from other trait than their dietary and reproductive habits.