And people are responding in kind that someone's not knowing something isn't the baseline definition of "Spoiler,"
Yes, it can (and should) be taken into account to varying degrees, but there's also a point at which the person who doesn't know whether or not they've actually been spoiled need to apply perspective to the situation, and examine the context. This thread is as large as it is because there are a lot of people who get to that point, and decline to apply perspective and context, and stall out at "But I didn't know!" as if that should count more on a discussion forum where knowing what you're talking about is maybe not as valued as it should be, but it IS definitely valued.
Yondu isn't in any trailers, either. I don't think saying his character is in the movie would be considered by anyone to be a spoiler - at least not until someone who didn't know any better decided to get publicly aggrieved at the idea that there are things they don't know about that other people DO know about and that's unfair.
If a guy goes to the trouble of looking some shit up to see if and when a piece of information is public knowledge to a certain degree (as the Thanos information absolutely was) then it's hard to say they're being completely inconsiderate. They're thinking before they post. But if you are suggesting that discussion of non-spoilers needs to be hampered and further constrained because there are people who DON'T KNOW that's a NON-SPOILER and that in itself is a SPOILER, then we're going to continue to go in circles because that's kind of ridiculous.
Again - if you're making the decision to go above and beyond when it comes to that first viewing "purity," then at some point you need to take that into account and behave accordingly, and that doesn't mean getting angry at people when they don't spoil a non-spoiler that you didn't know about. It's on you to discover whether or not you ACTUALLY got spoiled before you start complaining, not just react. Otherwise, the inconsideration and unfairness is on the part of the person who does not know what they're talking about dictating the terms of discussion for everyone else.