Soylent Green isn't as well remembered a film as others on this list. It falls in the category of confusing seventies sci-fi that filled the gap between 2001 and Star Wars, but with few highly regarded entries. With Charlton Heston playing a detective this cautionary tale tackles overpopulation and big business (not to mention euthanasia) in an smart and assured way.
Already in this list I've leaned on the argument that the twist is so ubiquitous that it would be ruined sooner or later. And while this is true to some level for Soylent Green - yes its a minor film, but the twist has risen beyond that - theres a key reason behind this case; the eternal Simpsons did it.
The Simpsons was massive and getting a reference on the show in mid-nineties was an acceptance of your eternal place in popular culture (now it just means you were noticeable for a brief period). Theres numerous mentions to Soylent Green throughout the shows future-set moments (as well as recurring elements in Futurama). Once somethings been done on The Simpsons it no longer becomes spoilerific. Which would be true of Kane, if the Rosebud episode had explicitly said the film.