Is there a certain timeframe in which a spoiler warning is no longer necessary for a work that is old enough?
There's the very common spoiler that Vader is Luke's father and no one really needs to use a spoiler tag for this.
I ask this question in direct reference to this answer that spoils a key plot point in Orwell's 1984. This novel was written in 1949, 30 years before Empire Strikes Back, and the novel is a very common selection in most high school English curriculums (at least it is in the USA).
Is a spoiler for something like this still necessary? At what point could I say to myself, "I'm referencing a story that is old enough, I don't need to place a spoiler tag here."?