The concept of a "plot twist" which can be "spoiled" is a fairly recent concept in the history of drama/literature. In Ancient Greece, for instance, everyone knew the all of the legends and their plots forwards and backwards - if you found someone who didn't know that Klytemnestra killed Agamemnon, you'd think them ignorant and remind them of the story.

Or take Shakespeare's plays - Iago and Richard III explicitly detail their villainous plans to the audience, it's not concealed like the identity of the murderer in an Agatha Chrstie. In Elizabethan times, a "comedy" meant a play with a happy ending, just as a "tragedy" meant one with a sad one, so even before the audience sat down in the Globe they'd know that Romeo and Juliet wasn't going to end well for the lovers. Shakespeare even "spoils" the ending in the Prologue: "A pair of star-crossed lovers take their life."

Or take Robinson Crusoe, considered the first novel in English. Its full title is The life and strange surprising adventures of Robinson Crusoe, of York, mariner : who lived eight and twenty years all alone in an uninhabited island on the coast of America, near the mouth of the great River of Oronooque, having been cast on shore by shipwreck, wherein all the men perished but himself, with an account how he was at last as strangely delivered by pirates, also the further adventures, written by himself. So no one was worrying about giving away the ending, "he gets rescued by pirates" - it's right there in the title!

Literature developed, of course, and by the time of novels like Emma or Tom Jones we see dramatic plot twists, and in Barchester Towers (1857), we even have the concept of a "spoiler":

And then how grievous a thing it is to have the pleasure of your novel destroyed by the ill-considered triumph of a previous reader. "Oh, you needn't be alarmed for Augusta; of course she accepts Gustavus in the end." "How very ill-natured you are, Susan," says Kitty with tears in her eyes: "I don't care a bit about it now."

You are asking about centuries ago, but the phenomena of spoilers is much more recent, brought about by the internet age more than anything, I think.

Take this Variety 1960 review of Psycho. They were not exactly keen on keeping the biggest secrets.

Some highlights:

"throughout the feature is a mother who is a homicidal maniac. This is unusual because she happens to be physically defunct, has been for some years. But she lives on in the person of her son."

"Among the victims are Janet Leigh"

"Martin Balsam, as a private eye who winds up in the same swamp in which Leigh’s body also is deposited."

"the psychiatrist who recognizes that Perkins, while donning his mother’s clothes, is not really a transvestite; he’s just nuts."

This review (June 22) is 6 days after its limited release. Its wide release would not come until September 8th.

A follow-up question: To what extent were the storylines of Shakespeare's plays already known to the average audiences of his shows? I recall learning about a Greco-Roman work with a similar storyline to Romeo and Juliet, but I am curious whether such works had entered the cultural lexicon of Shakespeare's time or if there was even a link between the two storylines beyond coincidence.