We could stop thinking of them as walled gardens we're trying to get into and start thinking of them as quarantines for the Eternal September population.
Hacker News actually has a way to do this quarantine, a version that only counts votes from accounts created before 2008: https://news.ycombinator.com/classic. About a decade after Eternal September started, but not bad.
So that's at least strictly before 2009.
If there's something magical going on, the ordering would be the only real distinction possible.
Apparently the original (still the same?) dates back to the OG PG days:
Ask HN: who knows about this - "http://News.YCombinator.com/classic"? - https://news.ycombinator.com/item?id=1832884
It's what the frontpage would look like if only the votes of users who joined in the first year counted. I made it to check if the site is declining. In practice it's usually just a slower-changing version of the frontpage.
~ pgMaybe we should have a setting "count only votes before date yyyy-mm-dd" but this would probably make the site too dynamic (?) and consume a lot of resources.