You're right, but curiously every post announcing Rails 5.1 at the time got nowhere on HN: https://hn.algolia.com/?query=rails%205.1&sort=byPopularity&...

I think there's now too much going on and too few people reading /newest for the HN front page to be a resaonably accurate representation of the big stories if a key Rails release didn't even make it at the time :) Seen quite a few similar instances as well. (The real issue IMHO is not enough exposure of or encouragement to visit /newest, so it's pot luck if links get seen.)