I'll say it right up-front: this is not an argument against cooperating with people who are trying to avoid spoilers. It's obviously a good idea to avoid spoiling people who don't want to be spoiled, so long as such people are sufficiently prevalent in the population that the minor inconveniences of spoiler warnings etc are predictably going to be appreciated.
Epistemic status: devil's advocate. I've been unsympathetic to spoiler concerns for a long time, for more or less the reasons argued here, but my arguments are far from perfect. Should they be convincing? I don't know. I still occasionally avoid spoilers, but not usually.
For an ideal agent, information should not be harmful. This is violated in many ways in practice, but these often point to irrationalities (including imperfect decision theories, like evidential decision theory). We should avoid harmful information when we know we can't handle it, but I also strive to be able to handle information of all kinds.
As JenniferRM mentions in the comments, avoiding spoilers has a chilling effect on discussions about the intellectual content of media. This means that embedding ideas into fiction can actually inhibit their spread in a weird way.
So, why avoid spoilers?
1: the puzzle argument.
Obviously you don't want a puzzle spoiled, right? It deprives you of the joy of solving it. You can't learn if you don't at least try to solve it yourself before getting the answer, right? Spoilers for movies, TV shows, and books are like this too, to the extent that the story presents puzzles. Even if you're just trying to enjoy yourself, learning and enjoyment often go hand in hand -- the reason your brain rewards you for engaging in play activity is because it satisfies evolutionary heuristics for learning (probably).
But is it true? Do we need to avoid spoilers, solving things ourselves, in order to learn?
First, let's consider an idealized learner.
The perfect Bayesian can learn just as much regardless of what order the evidence is presented in. It's not literally true that the only way to learn to do something is to struggle-fail-revise-retry yourself before getting advice. It's not even necessarily the best way to learn. We learn lots of things by being told. (Would you prefer teachers keep Darwinian evolution secret, waiting for students to invent it themselves? An extreme anti-spoilers mindset would suggest something like this! Indeed, a friend of mine lamented being spoiled on all the big important ideas like evolution, relativity, etc.)
Rather than avoid spoilers, I would prefer to develop the skill of "un-seeing", to be able to inhabit the ignorant mindset and model what mistakes I might make, what wrong paths I'd go down, what strategies would be fruitful vs fruitless. (For example, we could put ourselves in Darwin's shoes, and imagine the way he had to think in order to come up with his theory.) I do think we can develop a skill like this to some extent.
But let's consider less idealized learners.
For machine learning, at least, imitation learning is much more efficient than reinforcement learning. For reinforcement learning, you have to find good paths and then learn them. For imitation learning, you're given the paths; you just have to learn them.
Humans are pretty good imitation learners. Children pick up lots of things from those around them.
The full skill of "un-seeing" is pretty difficult for us -- it isn't easy to inhabit the ignorant mindset once we understand something. However, good teachers like 3blue1brown don't just explain the solution, they explain how you might have come up with it yourself. Essentially, a good teacher does the un-seeing work for you, so that (by imitation learning) you don't just get the solution to the one problem, but are actually a little better at problem solving more generally.
Now, it does seem very helpful for humans to do a little reinforcement learning to cement that knowledge in place: to do it yourself rather than only ever watching other people do it. But (pedantically, for the devil's-advocate argument this post is making) I want to point out that a great way to do this is to teach someone else, making you go through the demonstration in detail for them. So you don't necessarily ever need unspoiled problems to learn on.
Indeed, the skill of teaching and the skill of unseeing are very closely related.
"But you should at least test your skill on un-spoiled puzzles, even if that's not how you learned" says the one.
Maybe so. But if you really understand something, at the S2 level (not just the S1 level), you can see at a glance that you know how to solve a puzzle, without actually solving it. If you need to test, then at some level, you lack understanding.
(Which is fine! But notice that this is not a necessary thing.)
It's like people have a concept of reinforcement as an important type of learning, but it's making them think that they have to stub their toe at some point, otherwise they'll never learn to avoid furniture properly. You don't necessarily have to personally stub your toe. Trying problems is a pretty good way to test how "fake" your understanding is if you're unsure. But there are other ways to notice this, and other remedies. It seems to me that people put trial-and-error learning up on a bit of a pedestal when thinking about things like this.
2: the intended experience.
Even if we set aside the puzzle thing, the author probably wrote with the un-spoiled experience in mind. Getting spoiled changes your experience. You can only get the un-spoiled experience once; after that, you can get the spoiled experience as many times as you want.
First off, I almost always read/watch things just once, so this argument isn't very persuasive for me. I just have a choice between the spoiled and un-spoiled experience. Which is better?
“It turns out even halfway through a story, you enjoy a spoiled story more, before you get to that spoiled ending,” said Christenfeld.
But that's not my real argument.
My real argument is that I haven't experienced things being worse due to spoilers. I have had my experience of a book harmed by someone telling me about it, but only when they tell me incorrect or misleading information which makes me anticipate the wrong things or misunderstand. On the other hand, knowing a twist ahead of time and anticipating it, and getting to see just how it unfolds, has been enjoyable.
Don't get me wrong -- I've also enjoyed a really good unexpected twist in a movie. Certainly I couldn't have had that exact experience if I had known more about the movie. But there's this other experience, gleeful anticipation of an upcoming twist, which you can't have without spoilers.
Also, if there's something really good about a story, it's usually worth knowing about -- whether or not I will read the whole story. Since I don't have time to read everything people mention, I'd rather people summarize it for me if it's really interesting, so that we can talk about it right then rather than waiting until I've read it. Recall my earlier argument that enjoyment somewhat tracks learning. To the extent this is the case, total enjoyment should be preserved no matter how you learn the information. So if you can enjoy lots of stories a little bit by getting spoilers for them (and only ever reading/watching a fraction of them), that would be more enjoyment total.
(As with the skilled-teacher argument in the previous section, this enjoyment depends on the storytelling skill of the person who spoils it for you, of course.)
That's my experience. Obviously, things could be different for you. However, many readers may have never really asked themselves the question. Is your experience really ruined by spoilers? Or is it just different? Is it worth the trade-off of the experiences you have to lose out on in order to avoid spoilers? Are you avoiding spoilers because it's good for you, or because it's just what everybody does?