[harabat]

Seems like FriendlyCaptcha [0] might tick the boxes. Here's the author talking about it [1].

[0]: https://friendlycaptcha.com/

[1]: https://news.ycombinator.com/item?id=24921288

[charcircuit]

FriendlyCaptcha is not even a captcha. It's just a client side rate limiter.