Oh, I see what you're saying. You're right, it's not super common, but I've definitely seen it. For example, it's fairly common for people (people in this case meaning "I do it, so I must not be the only one!") to spoiler-mark something if it JUST came out so someone who hasn't quite realized won't be spoiled before they watch it.
And even ignoring that, the whole thing sort of falls apart when someone spoils something about an episode an hour before it airs, and then someone else spoilers something about the manga fifty chapters ahead in response, and etc etc, then they go back and read and oops. There are plenty of potential situations where someone might be spoiled on something years away without expecting it.
Basically I think it's kinda rude to keep talking about the manga when people have asked for that to go away.
Personally, I... I have no problem being spoiled. My opinion is that if a spoiler ruins something, it wasn't good to begin with. But I know a lot do care, so I try not to do it!