Wow, I didn't know about it. Big thanks for this suggestions. I will add this filter soon. What do you think should be a name for this filter? Maybe something like "old posts"? :)
My suggestion would be to comingle them with the Show HN submissions; they’re the same thing with a different way of phrasing the title. If someone wants to look specifically for old posts, you have that covered with the year filter (where the reader can choose 2009, 2010, etc.) Also, note that a few submissions begin Ask YC: Review {our,my}....