I've seen multiple comments suggesting HN uses ReCAPTCHA, but I have never encountered it myself, and I even have Javascript disabled and login through 'anonymous' IPs such as tor, so I'm unsure what these users could be doing that is 'worse' to trigger ReCAPTCHAs.
If most users don't even know that ReCAPTCHA is used, that's a good sign that it is being used as little as possible, though.
I've seen it before. In fact, I'd double checked last time I brought it up and it was there on /login in an incognito tab.
Either HN has changed or they now conditionally load it. For example, I encountered it every time I used Tor on HN, though I haven't hit /login in months.