In battle, Pokémon are basically indestructible. No matter what kind of attack they endure, the worst that can happen to them is they’ll faint. A slash from a Scyther won’t sever your Caterpie in two. A punch from a Machamp won’t shatter your Rattata’s bones. If this wasn’t the case, then it would be impossible to ethically justify battles.
However, there are also many circumstances where Pokemon are depicted as being susceptible to injury. For example, I recently watched The Power of Us. In the movie, we learn that the old woman’s Snubble died as a result of a fiery explosion…but why? If every Pokémon can endure a Blast Burn from a Charizard without being reduced to a pile of ash, then why would this explosion kill Snubble?
The obvious explanation is that Pokémon are only capable of being harmed when it’s convenient to the plot, but that’s boring and terrible. If you had to come up with an in-universe explanation, what would it be? Why are Pokemon indestructible in some circumstances but not others?
I kind of suspect that this is actually part of Pokémon training – learning to use your attacks accurately, under pressure, in a wide range of situations, and non-lethally. I mean, that’s part of martial arts in the real world; you have to be proficient in not just inflicting maximum damage, but also in inflicting exactly the amount of damage you intend to and no more. In real combat sports, if you’re in a match and you kill your opponent by mistake, you generally have to flee Los Angeles with your petite French girlfriend and your father’s precious gold wristwatch, and I don’t think most Pokémon can even drive a stolen motorbike, much less rescue a mobster from a sex dungeon. The point is, there is a certain amount of control and holding back that is probably exercised in all but the blackest of underground cage matches.
(This is actually explicit in the unfinished but excellent Nuzlocke comic Alterity, which should be required reading for anyone who wants to talk about ethics in the Pokémon world)
You wouldn’t expect wild Pokémon to hold back like this, but I think the amount of time we spend actually fighting wild Pokémon is subject to a certain degree of wiggle room. Like, in the games we might steamroll them by the dozen while training, but in the anime we rarely see anyone fight wild Pokémon to train (and if they do, the wild Pokémon are often on board with it). Normally when you’re fighting a wild Pokémon it’s either because you’re trying to catch it (and it has accepted this as a challenge) or because it is, to all appearances, actually trying to kill you. Meanwhile in Let’s Go Pikachu and Eevee, with only a couple of exceptions, we don’t fight wild Pokémon at all; it’s Pokéballs or nothing.
I’d also like to relate this to some other ideas I’ve had before about why Pokémon can learn only four moves – “learning” and “knowing” a single move actually represents a huge amount of behind-the-scenes practise that allow you to use it in a dizzying variety of possible situations against any conceivable opponent. Well, here’s another reason it’s difficult and complicated: you also have to possess the skill and control to ensure that your moves will never be lethal. Honestly, it’s kind of impressive that they can even manage four.