The ferishizstion of spoilers is very modern. The very term dates from usenset I believe. However, the first movie that the had a mainstream "don't spoil" movement I can remember was The Crying Game. The meme at the time was "Don't tell the secret!" and "What is the secret of The Crying Game?" (Note the lack of the term "spoil.") The secret is that the girlfriend is trans. (There. I "spoiled" a 25 year old movie you probably had never had any intention of watching, that is really only notable for "the secret" and the socialization around it for a few months in 1992.)