I could see spoiler tags being relevant for like...two weeks at best? But the issue is that what constitutes a spoiler differs from person to person. Any new work that ever releases would qualify for different people, but having a spoiler tag exist on every work for two weeks feels like it would be awkward. Not to mention that while some series instantly get posted here, others are ignored for months or even years before they get fanart.
It made more sense when the site was newer and you had people worshiping the ground Touhou/Madoka walked on, but with how big the site has grown I think it has run its course. You can't play any form of online game without someone crowing about series spoilers on everything from Full House to Spy X Family, not to mention series spoiling their own twists inside their newer media. (The Fate section is particularly funny about this.)
Could probably get rid of the spoiler designation entirely and nothing would be different. Half the names that people demand to be spoiler-statused are nearly a decade old if not longer, and you can't preserve that status forever.