Though one of the first uses of the term “spoiler” in this sense was apparently documented in print only in 1971 [Wikipedia], the concern that plot elements shouldn’t be given away if they would undermine suspense was very much an idea more than a decade earlier.
In her March 1960 “Reader’s Choice” review of Colin Wilson’s Ritual in the Dark in The Atlantic (page 114), Phoebe Adams expresses the passively threatening sentiment that “it ought to be a hanging offense to give away a plot that an author has carefully arranged to keep the readers’ curiosity on the boil.”
Although I, Luddite-like, was reading this in a print copy of the magazine I came across very much offline, the article is available in digitized form.
There’s more, incidentally, on Adams in a farewell interview in the August 2000 issue, and more of her writing is in the extensive and well-organized online archives at theatlantic.com.
Colin Wilson [Wikipedia] “also wrote widely on true crime, mysticism and the paranormal, eventually writing more than a hundred books.”