For me, it isn't that someone headcanons "so and so" as a bottom. It's more that generally the more feminine (or nicer) character is always the bottom.

It's kinda interesting to see people always argue about top/bottom dynamics but rarely do you ever get interpretations that have top/bottom/dom/sub as quadrants where someone can be a bottom who switches dom/sub based on how they feel that day, or a sub that switches top/bottom based on what the dom wants, etc.

I just roll my eyes at anyone getting worked up over it in either direction, trying to police how people write their fanfic smut is annoying as hell in general. And I'm someone who for the most part doesn't have a preference with who's the top/bottom with my m/m ships but if I see someone trying to tell others what they 'should' be writing, even if it's an 'x-as-bottom' that I like I'll block them on sight. I wish more people would admit that when they prefer one character as the bottom it's because they find it hot, not argue how it's more 'in character' or 'progressive' like what?

And yeah OP a lot of people in fandom talk like they've never spent much if any time around queer men who, not always, but often do have a preference. This doesn't mean they never do anything outside of that preference ever, but most of the time they'll fall into it because it's what they like.

I've come around to understand every individual's kink is personal. I try my best not to yuck anyone's yum, but I do get annoyed when my kink for a character or ship is met with, "Character A isn't a bottom/sub!! You're wrong!" (Or vice versa)
I'm so tired of having a prepared 5 page PowerPoint program to explain my kinks. Mostly because those who demand I defend my preferences don't even want to listen. They just want to talk over me and poke holes into my preferences.
So I won't stop anyone from talking about their preferences but I likely will never talk about my preferences to anyone unless I enter a space where my preferences are the norm.

I just want to know when YKINMKATO stopped being a thing.

I probably won't even notice a particular author having a preference for a particular top/bottom configuration, although I might notice if they do for dom/sub. But I will definitely notice if they always conflate dom=top and sub=bottom, especially if the characterization warps so the dom/sub stuff is 24/7 and not just about sex.

(Yeah, some rl people do 24/7 D/S, but most characters pretty obviously don't.)

Like, I get that having a canonical perfectly competent adult turn into a weeping helpless waif who not only wants to be stuffed with cock but can't actually function on their own and needs a big strong dom to tell them what to do is a kink that doesn't even need sex, but it squicks me way more than "character A gets off on tying character B up and making them beg to suck A's cock, then they get up the next day and fight crime/the heroes/corporations/intergalactic tentacle monsters together."

Especially when I start reading a fic tagged "A is a dom top and B is a sub bottom" that I assume is about sex, when actually it's about the characters having had personality transplants so now they're both unrecognizable.

I get people having preferences, I get people being enthusiastic about their preferences, I get people having reasons for why they interpret characters as having those preferences ("X is a leader, every day he's under enormous amounts of pressure, so I think he'd love being able to just give himself away for someone else to control and take care of him for a change").

I don't understand, and really dislike, when people who headcanon X as a whimpering sub bottom mock people who see them (okay, HIM, usually) as a top because lmfaoooo they clearly must be y/n girlies wanting him to be their dom daddy, too straight to realize his True Self (or some other "you're just too stupid to realize that I'm right" reason). First of all, psychoanalyzing someone based on their shipping preferences is lame as fuck; second of all, actually I liked to see X as a bottom at first, but seeing how you act about it made me defect to the other team and now I see him as a top 50% because it's hot and 50% out of spite lol.

Nah, I think they just didn't want to be critical of tourism because of their shiny resort in Hawaii.