To be reasonable, way more effort falls on the person who doesn't want to be spoiled than the person who has to tag stuff
For example, I haven't finished Horizon yet and I know nothing about what happens past where I am. I don't go to the OT or the Spoiler thread, or any discussion threads regarding the story. I don't go to related subreddits or search for gameplay
It's a two way street. One has the context and common sense to know what is a spoiler or not, the other has the common sense to avoid situations where they could be spoiled
In some way, I think "spoiler" is the wrong word because it gives the implication that it spoils/ruins the work. Like bad rot on an apple. But that's not how people in general regard or react to spoilers. They can still enjoy and appreciate the work, but that enjoyment is different than if they had no foresight because such knowledge changes context, expectations, implications, and so on
Which is as it should be. There's a reasonable level of civility and keep others free from spoilers.
If I know my friend is currently watching The Walking Dead, I don't spoil it for him. If a show just aired, trying to keep spoilers tagged or to a minimum. But there is the other end as well.
If you enter a GAF or Reddit thread about a topic, there's a certain amount of acknowledgement that you're joining a conversation, with folks who are more verse in the subject than you. They should be allowed to have a discussion on the topic freely. They do not have the full details of where you are, everyone is at different places, and spoiler tagging everything everywhere is untenable. It's why hard guidelines like Plum's example don't work.
As an example, if you see Sean Bean on an interview (technically, official material) and you're only on the first episode of Game of Thrones, you're going to get spoiled. That same is true of many of these spoilers casually dropped in this thread. (P.S. While I understand the point, don't do that, it's an unkind move.) Should Bean or others around have to talk about a specific narrative situation because you're only on Episode 1? No, that's probably not going to work.
It's a shared responsibility, use it well and do not attempt foist all the responsibility on another party.
You really see this as a weird "spoilerphobes trying to control the entire world's discussion" thing?
To be fair, we had an entire Zelda thread derailed because someone made a joke that folks thought was a spoiler. We had the derail in the Dark Souls Man at Arms thread pointed out above.