[thaumasiotes]

> Grishnákh, leader of the Mordor Orcs, accuses Saruman's Uruks of eating Orc-flesh, which they angrily deny.

As you note, this is good evidence that cannibalism is not practiced by the orcs.

> In Cirith Ungol, Gorbag suggests that Frodo (recently poisoned by Shelob) should "go in the pot"; Shagrat indicates that Gorbag could be "for the pot" for making such a suggestion.

This isn't good evidence; the insult is suggested by the context.

> Shagrat threatens to eat a disobedient orc

Without some sort of context, this is good evidence that cannibalism is practiced.

> and after killing Gorbag he licks his blood from the blade.

But this isn't; blood licking is not unknown among humans either. (To be fair, neither is cannibalism, but they read pretty differently.)

[firethief]

>> Grishnákh, leader of the Mordor Orcs, accuses Saruman's Uruks of eating Orc-flesh, which they angrily deny.

> As you note, this is good evidence that cannibalism is not practiced by the orcs.

Institutionalized cannibalism is characterized by strict taboos about who can eat whom, when, and how. It would make sense for an insult between two related cannibalistic societies to be based on either "you fail to observe the taboos we both recognize" (a heavy put-down) or "you don't recognize our taboos / you have specifically disrespected us in the way you violated a shared taboo" (insults with deep roots; a likely casus belli). In either case, angry denial seems like an orcish way to counter the insult without escalating a potentially volatile situation. So while cannibalism-related insult is consistent with societies in which cannibalism is never acceptable, among societies where the practice is formalized absence of such insults might have been more surprising.

[thaumasiotes]

If the accusation is just "eating orc-flesh", the only cannibalistic taboo that would be violated by that unqualified act would be eating those who aren't members of your group.

A practice of eating your own kind -- but not other people -- would be consistent with everything mentioned above, but would be incredibly bizarre for humans. We are much more likely to eat other groups while never eating our own.

Orcs could be weird that way though. It's not hard to lay a basic conceptual foundation for eating your own kind and prohibiting other groups from eating your own kind.

[firethief]

> If the accusation is just "eating orc-flesh", the only cannibalistic taboo that would be violated by that unqualified act would be eating those who aren't members of your group.

No. For example, it may be unacceptable if the groups aren't at war.

> A practice of eating your own kind -- but not other people -- would be consistent with everything mentioned above, but would be incredibly bizarre for humans. We are much more likely to eat other groups while never eating our own.

Then humans are incredibly bizarre. https://en.wikipedia.org/wiki/Endocannibalism

[thaumasiotes]

Nothing in the Endocannibalism page suggests that any mentioned group ate their own members to the exclusion of eating members of other groups. That topic isn't discussed at all anywhere on the page.

The most prominently mentioned example, though -- New Zealand -- was definitely not exclusive endocannibalism.

The exocannibalism page confirms that the Wari' also practiced exocannibalism.

[firethief]

The article is obviously very basic; I just linked it as a starting place for your research. This is not really the kind of thing where arguments from first principles are productive.

The exocannibalism page that you found does say that the Wari are the only known example of both types of ritual cannibalism, which suggests that it's more of an exception than the rule.