Considering the remaster just came out, causing a surge of interest of newcomers, you deserve a good chewing out. And your last line sounds incredibly self-centred, as if everyone should have the exact same experiences and it's their fault they haven't gotten round to something. Normally I wouldn't care too much, but your attitude towards this is deserving a a verbal smack. Have some common courtesy, it's not that hard to add a spoiler warning.
I say spoilers are more like how trademarks can become generic. It's not a specific period of time, so much as a general acceptance by the general public consciousness*. Easy cheat sheet would be to see if it was joked about directly in The Simpsons or other major comedies. The spoiler you referenced is one known mostly to people who played through the game and learnt it by playing through the game. It's laughable that you think it's on par with the other two. Side note, I somehow did get to Citizen Kane without any knowledge (even with watching The Simpsons' episode) and while it wasn't the deciding factor my adoration of the film, it was a nice little conclusion that I wouldn't want to take from anyone else if I could help it.
*not everything in a piece of media is a spoiler, but it's not that hard to infer what is and isn't, eg random sword in Dark Souls, not a spoiler, sword named after important character, likely a spoiler to a certain boss or piece of lore, so take the extra second to tag it. I also don't get people who hover around threads of things they're currently going through - you're on the fastlane to spoilers at this point. And spoilers shouldn't ruin a piece a media, but they can damper the enjoyment of them.