Like Math CAPTCHA, only deeper and with more subject matter versions? One of the math forums (Cut the Knot?) had something like this, that actually required (I think) calculus.

Reputation systems such as HN itself are another approach. These might not scale; or they might require an eigentrust-like system to scale; or scaling via eigentrust might create and reinforce the same kind of filter bubbles we see on social media. Who decides who’s a climate expert?

Steemit is an attempt at a blockchain-based reputation system. Whenever I’ve looked, it’s been dominated by low-quality content, mostly about crypto itself. Maybe this is growing pains, or maybe a reputation system needs a different go-to-market (that bootstraps from an existing platform or community), or maybe wedding reputation to crypto — or at least to crypto that pays out — just isn’t a good idea. (You didn’t exactly suggest that, either; I just riffed my way over to it.)