I think spoilers are timeless. It's stupid to me to think that people think it's "alright" to spoil a game just because it's been a long time since it released. "Stop complaining about spoilers for a 18-year old game!" - yeah, but what if I never played that game and want to experience it the way everyone did 18 years ago?
I completely agree with this. (I even regret coming to this thread, as the "common knowledge" zelda spoilers OP mentioned I did not know about. While not explicitly spoiled I still will view those aspects differently.)
Think of it this way. Every single day there are passionate gamers who play Ocarine of Time for the first time. Or any other old game, and if you just bring up a spoiler out of context it might ruin that experience for those yet to play it. Especially if you think the game is still worth playing, why would you spoil it for someone without warning? And even if you think that something is common knowledge, realize that there might be some people who don't know it yet, but would be interested in playing that game still.
For example I was just starting to go through the Zelda franchise and had been getting through the 2D ones, and was about to move on to Ocarina of Time and Majora's Mask. There are plenty of people who have not gotten to play many older great games, and especially if you think those games are good and worth playing, be considerate and put a spoiler warning. I personally always avoid spoilers (put spoiler warnings) no matter how old, and if I see a spoiler in some comment sections, I try to warn others to not read the comments that there are spoilers, etc. Because if I have managed to save at least one person from having a good game spoiled that they would still play later, it is worth it. And I appreciate it greatly the other way around as well.