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Bots can now solve CAPTCHAs better than humans
(calckey.world)
This is a most excellent place for technology news and articles.
Proof of work could be easily combined with this, if the wasted computational cost is deemed necessary/worthy. (At least it's wasted CPU cost, instead of wasted human time like captcha.)
Tor has already implemented proof of work to protect onion services and from everything I can tell it has definitely helped. It's a slight inconvenience for users but it becomes very expensive very quickly for bot farms.