I think it's frontpage ranked by only users from 2009 and earlier:

https://news.ycombinator.com/item?id=26465569 - HN front page ranked using only votes from early users (2009)

But a similar thread from 2009 says:

https://news.ycombinator.com/item?id=607271 - HN Frontpage ranked using only votes from accounts over a year old

I am not sure if the one year threshold is relative to current year, or fixed at 2009.