There are new features/changes added often, many are documented here (not with dates, but you can get an idea of when things change based on the commit history): https://github.com/minimaxir/hacker-news-undocumented
The most recent change I remember was when Twitter and GitHub posts were split based on the user the link belonged to (i.e. GitHub.com/dang is different than github.com/torvalds now).
