Using recent activity as a metric seems like it would age finished projects out of the github platform. Active projects that have switched version control several times in their history might refer to previous versions on github without github itself being aware of the connection.

At the free level I could see unmodified forks and anything marked private disappearing over time, then culling empty accounts. If I created an account, forked a project, and then left, how many months would need to pass before I'd be surprised that my account hadn't been recycled without my active consent?