The thing about "If they don't want to be responded to, then they can just say nothing at all" is that people can and will reply to you well after you actually made whatever post because the post is still there. Not every thread moves at lightning speed, and even when they do people can reply to a thing that catches their eye late. So a post can tell people "Oh I shouldn't quote this in my response" and thus lead to less quoting, and thus is an effective strategy in disengaging. Also, there's a thing where addressing someone's points after the fact or ideas that they brought up is one thing, but people will reply to a post that says "Please leave me out of this" with something addressed to them personally. Whatever you think about last words or whatever else, that doesn't meaningfully extend to cover a personal response.
One time I was the second person to reply in a thread only to realize that actually I really did not want to continue being involved in that thread because people were saying very upsetting things, so I made a post where I explicitly backed out. Because otherwise as post number three I think there would have been a fair amount of engagement, which I didn't want. The wonderful thing is that this worked and, as far as I can tell, nobody cared? If people wanted to respond to my ideas, they could and presumably did, just without trying to talk to me.
I don't know why people are so firm about categorically rejecting an onus to accommodate other people as not, like, your job. What do you all think "Be courteous and decorous to everyone you deal with." means other than assume more responsibility for other people's wellbeing and comfort in your actions than you have to? What the fuck is civility if not that? You understand, it's not about whether there's a moral imperative, it's is it civil? And it's clearly not. Like, continuing to talk to somebody who has asked you to leave them alone in any real life context is clearly being disrespectful and like an annoying little kid. The fact we're writing it down, doesn't change that. This is stupid. This is so obviously stupid, I legitimately do not understand how people can think that it's civil, except by replacing "Be civil" with some other conception of like Proper Debate Etiquette. But that's not what it is. It's civility. It's courtesy, it is literally going out of your way to be nice and respectful of other people. That's, that's the thing that it is. It's extra obligation to everyone else that the forum is saying that you have. Obviously.
Again, if you want to talk about something somebody brought up after they've left the conversation and asked not to be responded to, you just talk about the thing they brought up? You don't need to talk to them. So in no way does "Don't respond to people asking you not to" curtail a discussion, because it doesn't give them carte blanche to stop conversation dead. But neither is it a meaningless silly thing only babies do that nobody should care about. You just keep talking, but you don't address what you say to them. It's the easiest thing in the world.
I cannot stress enough, I'm having a genuine cognitive empathy failure moment here, I cannot fathom how it is anyone disagrees with this.
Like, people make disengaging posts to say "Please don't respond to me", because getting responses can be upsetting, or frustrating, or simply tempt the person to respond again when they would rather stay out of the conversation. Because when someone responds to you there is often a very strong and natural desire to respond back. So they may go "Hey, I'm gonna drop out now" because they've realized continuing this conversation is not what they want to do and announcing that makes it easier for them to stick to such. Or they may do it simply to provide an air of finality to it so that they are more likely to stop. And these are completely rational actions, because people have emotional responses and acting to better help your future self make the choices that current you knows are best means you'll be more likely to make good choices, which is in your self-interest, which is thus rational to do.
Describing what is basic etiquette children learn as being guardrails in case other people are irrational is something that I think really speaks to people losing sight of what the whole point of the rule is: You are talking to people. Real people whose feelings matter and matter much more than whatever forum argument you're in. Just... What mindset is this? Even if 90% of the time people disengaging aren't bothered, which is a figure you made up, because you don't know that people coming back to an argument means they weren't upset in the interim—even if that's the case, you realize "This behavior is only problematic ten percent of the time" is not a ringing endorsement, right? Like... Right? Am I crazy?