I recently posted a thread about an old movie from the 1950s (12 Angry Men), and provided spoiler warnings. More than one person replied jokingly that they were grateful for the spoiler warning for a 70-year-old movie. I've heard the same comment in one form or another many times over the years, and I really don't get it.
What's the expectation here? That we're all LLMs who've been trained on every movie released prior to 2010? It would be literally impossible to watch every film - even excluding obscure or foreign films - that humankind has produced since the beginning of cinema. I'm a huge movie fan who watches 2 or 3 new (to me) movies a week from pretty much every era, but I had only watched this very famous movie from the '50s in the last year, because I'm not a magic space baby with a brain containing all of the film scripts in history. The more films that are made every year, the less they will be watched by future generations, because time is a straight line and we haven't figured out how to pause the fucker yet so we can all catch up on 100 years of film.
I'm grateful that this old movie hadn't been spoiled for me, because I wasn't even an itch in my father's nutsack, nor he in his, when the film was first released. But the jokes in that thread would seem to imply that I would have had no right to be annoyed if the film had been spoiled for me, because... what? I should have had the good sense to be born during the depression instead of the '80s? I should have a working knowledge of every story every told prior to my birth? The fact that this very famous and very old film hadn't been spoiled for me shows that even very famous and very old movies don't automatically weave themselves into the fabric of your reality by the mere force of time itself. I had no clue what the movie was about beyond the very basic premise, because even spoilers for old movies are hard to come by when there're so many movies in existence. The jokes would only make sense if the opposite were the case.
If you care about spoiling films for other people, then there is really no time frame for a film's release that makes it 'fair game'. People have varied and unpredictable lives when it comes to the media they've consumed, and more often than not they're busy watching the current output of Hollywood rather than watching their grandparents' favourite films featuring actors who are all long dead, and before colour image was even technologically possible. The noble spoiler warning should be eternal.
And all of the above also applies to novels, plays, TV shows, video games, and anything else where spoilers might ruin one's first taste of it. Spoiler warnings are free, but they can conjure great cultural value seemingly out of thin air for those who are protected by them.
For #2, I'd definitely say it depends on the type of spoiler. Merely giving away the general events isn't going to ruin good media. Though if someone is basically in effect recreating the whole scene(s) lazily in a different medium, then that can ruin the ability to experience it in the same way.
I would liken it to how the Red Wedding in GoT was portrayed in its own media: As done in the earlier seasons, it was a crazy surprise of an experience. If it happened in season 8, it would've been a shitty, "here are the things that happened" kind of portrayal and it would've been kinda' what ever.
That's not even speaking to spoilers, but story telling in general. If they're doing a shitty job of telling the story, spoiler context or no, then it'll kinda' ruin the story either way. Spoilers are themselves story telling. It's not all black and white good or bad. It's how it's done and what is covered that makes it good or bad.
Though with spoilers, it's far less likely that people are going to try to really do it justice, especially in a meta-context like talking about the show, so spoilers are more often given in poor form for story telling.