I’m not mad about it. But I don’t think the time a piece of fiction has existed is a good determining factor for when conversation about it should casually discuss it without concern for spoiling it. Most people haven’t had the opportunity to experience most fiction for the entire length of that fiction’s existence. (Presumably, for instance, most people did not catch the Iliad during its initial run.)
I think social saturation is a much more relevant metric (for which time is certainly a factor). At a certain point, some works become a part of the social fabric that the mere concept of a surprise twist cannot apply. The Iliad is probably that. There is probably no one who could possibly read that work for a first time without already knowing how it ends.
Star Wars is not that.