If you use Tor and you view the headers on your requests to SN, you'll see a response header called onion-location:

onion-location: http://snsnsnya6h3ot563f3p566wuhfoklkg5f62hokdlaqzcaub3gf4xlxyd.onion

This results in TorBrowser (and Firefox and some others) offering ".onion available" in the URL bar next to the clearnet address you're visiting.

Yet for months now, when I attempt to visit stacker news's onion, it doesn't work:

Onionsite Not Found

Details: 0xF0 — The requested onion service descriptor can't be found on the hashring and therefore the service is not reachable by the client.

Why?