I think no spoiler warning should EVER be required for any game older than 20 years. I'm sorry, but by that point it's not his problem.
YOU GUYS (not just spenoza, of course
) ARE SILLY!
FACT: There is no "statute of limitations" when it comes to literature, film, video games, etc.
AS A COURTESY, it is nice to provide a disclaimer when revealing spoilers. Now, this issue is moot for most games in most genres, BUT for games that have neat little twists or compelling story, well, DAMN, provide a disclaimer. It's not onerous or labor-intensive to put a disclaimer at the beginning.
STATUS: I still love you guys
.