Talking about something hyped and new without regards for spoilers in an open setting is impolite. I don't expect to see someone discussing Dumbledore's death in the Random Crap thread two days after the book releases.
Deliberately spoiling things people have or may have been looking forward to is an incredible dick move - going around shouting SNAPE KILLS DUMBLEDORE at the people waiting in line to buy the book justifies you losing all your hair and muscle tone before your 30th.
There's a huge difference between a big spoiler that may really change the interpretation, the pacing, the reading of the source, and minor stuff. "Jabba the Hutt isn't even humanoid! He's, like, a huge slug!" is technically a spoiler, but won't impact your enjoyment of the movie at all, and people shouldn't get mad about it.
On the other hand, demanding people avoid spoilers for older media is also incredibly self-centered and asinine. I'm sorry, by now, "Snape kills Dumbledore" isn't a spoiler anymore - or if it is, at most in the sense that "Santa isn't real" is a spoiler - avoid saying it around kids. Yeah, I get spoiled too - I still haven't seen season 8 of Game of Thrones, but I have a pretty good idea of what happens. Where the line is? I dunno. A few months, a few years,... It's flexible and depends on the type of medium, and the societal impact of the thing in question.
Spoilers in general can be a minefield, and both sides can be idiots. try not to lessen other people's enjoyment of their media of choice, simple.