Researched it substantially and realized it's an unsolved problem. Anything that makes a dent is incomplete and comes with ugly tradeoffs. For a time I wondered if I should try and solve it myself, but I could never think any solution that hadn't already been/being tried. Years later I'm left curious if it's even possible to solve the problem.
My point is that captcha won't solve this, and solving this problem is a lot harder than it seems at first, and might not even be solvable (which I know is hard to accept).
If someone does find an elegant privacy ensuring way to solve it, I think the impact would extend far beyond HN and could make a big difference to the future of civilization as a whole.
Even if you use state ids for it, who's to say that a particular state won't be...loose with issuing ids that can then go on to be used for bots.
It's even a problem with humans as well - one human can be having a pleasant conversation with the other, not aware that that person isn't being genuine, or is lying, has ulterior motives or has been instructed on what to say by someone else.