Ah Shakespeare maybe but eh Agatha Christie is a bad example. Her plot twists are part of the story. She writes fricking mysteries for god sakes. So yes I think it would be really douchey to spoil the ending of one of her books just because they have been out for a long time.
And no unless you have read them I don't think those endings are common knowledge at this point like "Luke I am Your Father"
I think if it is a plot twist that isn't part of our pop culture (like the above Star Wars scene) then I wouldn't spoil it, I don't care how long it's been out.
If it is just some detail in the book or movie well then that is a different story.