No it won't. Unless the author confirms the plot point, it's not a spoiler, just a fan theory. No other reader is obliged to agree with how someone else thinks a story will end and having a plausible theory in mind will likely just make how it actually unfolds that much more satisfying. The thing is that, lot of times the audience will guess what will happen, but they won't guess the mechanisms through which those plot points are met, meaning that there's still space for them learn something new.
I also want to add something separate from my OG posts:
Predictions are not necessarily spoilers.
So someone guessed your plot twist and it's right -- ok. Other people don't know that. Only you, the creator, know that, and they're only right if you decide to tell them they're right early on as opposed to just...letting them guess and see if they're right.
Not to be rude (but I'm gonna be blunt and rude) the reason why a lot of creators don't and shouldn't care if the big plot twisted is guessed is:
If all your story has is a big surprising plot twist and once that's spoiled there's nothing else to keep your audience invested, it's a shitty story.
If the only reason people are reading/watching/viewing is to be surprised by the plot twist and then if it's spoiled the entire story is boring, that means your characters are boring, your world is boring, you plot is boring if it didn't have the twist and you have created something that has, at most, one reread/rewatch to see the foundations and go "ooooh now that makes sense". While a good story, I can and do rewatch/reread millions of time even knowing they're all dead all along/the monsters are really huma/everyones an alien vampire because the plot twist is just one piece of a bigger, more riveting story that I can enjoy even knowing the twist, and I would think most creators here are trying to create a good story and not just a giant plot twist with the illusion of a story around it. It's the same difference as creating a good horror film, or creating just a string of jump scares and going "well it scared them".
Well there is more ways to enjoy a story.
In most stories it isn't that big a secret that the hero will win and don't die. That's like a everyday spoiler the audience know. But they still look forward to see HOW the hero will win and how the hero won't die.
So if someone guessed that your hero won't die and that's the polt, just play along with something like:" We don't know that yet, and if that's true.. how will the Hero manage that?"
I'll repeat myself - there's a lot of READERS who think that way.
Hence why there is always so much drama, hate and screeching around spoilers to the popular franchises. I remember it was quite popular back in the day for trolls to troll people by posting gifs and screencaps from the camrips of Avengers Endgame, and how much anger, cursing and even legit death threats it reliably generated. You go try to tell those people that they shouldn't care about spoilers.
Hm, I don't think this has happened in my main comic yet. Though there was a time when the amazing @Azifri asked multiple questions that were answered literally 1-2 updates later. 8D
If someone guessed a plot line, I may play along, throw in fake hints or ignore it. Depends on what it is. Actually, a commentor did predict something and I did just ignore it one time. lol
I'm also surprised at some things people have NOT commented about/predicted yet. There was one thing I thought would be very obvious but nobody noticed. Once we get to that reveal, I think people will be like "OHHH". But it's not for a long time yet.
I don't think that parallels to what @HGohwell was saying. The MCU is vast there was a lot keeping people invested in that storyline, not just what happened in endgame. I think what they're trying to say is that if a story only has one big plot twist, and not multiple albeit smaller revelations, then the story lacks substance. There should be more than a possible plot twist to keep people invested.
And no one said people shouldn't care about spoilers, There aren't many people who want their investment to go to waste in an offhanded, unsatisfying way. But, if fans come to the right conclusion and talk about it, it's not a spoiler.
If they find out, just accept it but don't confirm them to be correct, that would just expose it to everyone else.
Seriously, many author's biggest worries (including myself) is if the readers who are so invested in their work would find out but should we change it to prove them wrong?
The answer for me is "No".
Because if we do that, we're going to have start everything from scratch to avoid so many questions.
Even if they know they are correct and we continue without noticing it, then this will at least give them a satisfaction IMO.
I put both 'confirm the guess' and 'do nothing'.
It may sound paradoxical, but what I mean is if possible, I'd confirm the guess to the specific person (privately - so only if the possibility is there and the person is fine with it), but would not confirm it publicly.
This being said, my story has multiple subplots rather than a big one, and has a huge lot of foreshadowing for each subplot, so I'd say, people guessing things in advance is rather unimportant, and almost reassuring. I'm more worried that readers might get lost!
I'm answering as a reader: I feel there is no need to worry in these situations. The fact readers may be lead unintentionally the wrong way for a little bit, is, in my opinion, a welcome consequence of having a rich, not too linear story. I find it very immersive, because that's pretty much what happens all the time in real life: a lot of guessing and imagining relations on things that are just random chance or unrelated events.
There is a vocal minority of readers that think that way.
What happened in Endgame wasn't a plot twist. It was, as the title suggested, an ending. A real plot twist ending is something like Shutter Island or Sixth Sense, perhaps weaker but still sorta The Wickerman. Perhaps I remember wrong, but nothing I saw in Endgame was something that dramatically shifted the plot and made me look at everything that had happened so far differently. It was a painful ending, sure, but there's difference between general spoilers and plot twist spoilers. And Endgame also had, what was it in the end, 20 something movies with these characters building connections with them and hence not relying on a giant plot twist to make it interesting so actually sorta proving my point.
Plus, you didn't ask about people posting spoilers of your ending, you asked about people commenting on big plot twists in advance. There is a difference between general spoilers and plot twists. Yes, people care about spoilers, but I bet lots of those people still went to see Endgame. And how many people still watched Madoka Magica even knowing the twist premise? If anything the plot twist drew people into that.
I'm saying, once again, if knowing the plot twist ruins the entire story it wasn't a well constructed story to begin with. It's fur coat and no knickers. I would happily spoil one of my big upcoming plot twists right now and still be confident most of my readers would stick around.
Do nothing. Banning them, erasing comments, and gas lighting them will just make you look like an asshole.
I had a short comic which someone guess the twist at the end and I was sort of happy because it meant they were paying attention. Having set ups to your plot twist is important, even if it is a surprise, there should be evidence to back up why it happens. I was able to guess the plot twist in the infamous film The Boy because there is a scene where the MC knocks on the wall and it's hallow. This is also why I think the plot twist in Frozen isn't good.