Perhaps just a simple linear decay function for every upvote would work. Every anniversary, a vote would lose 20% of its value, so after 5 years, a vote would e

Perhaps just a simple linear decay function for every upvote would work. Every anniversary, a vote would lose 20% of its value, so after 5 years, a vote would expire, unless re-confirmed by the original voter (or the voter could upvote a newer, different answer).