It hardly spoils the entire rest of the story. Star Wars is not about “what ever happened to Luke’s father”. If I had to pick between the prequels pretty much as they are, only with the Vader part cleverly edited out, and a genuinely good set of films that “spoil” the fact that Vader is in fact Anakin, I wouldn’t hesitate for a second. If I could have the best of both worlds, then it is also an easy choice. The reveal is still a major moment for Luke, therefore the audience still cares, even if it doesn’t leave quite the same impact on them as it used to in 1980.