Today, CAPTCHAs serve a similar purpose, except they’re used to train self-driving cars’ image recognition AIs. I always try to be a little subversive and correctly identify the images that are clearly unambiguously classified by the AI, and then purposefully screw up identifying the image that the AI struggles with. It lets me through the majority of the time, which indicates that my bad input made it into their training data.
Unlike the CAPTCHAs of yore, when machine vision simply was not advanced enough to solve them, anyone has access to pre-trained vision models easily capable of identifying the unambiguously resolved buses or crosswalks in the CAPTCHA image. The deterrent to spammers is no longer that actual humans need to solve the CAPTCHA, but rather that it’s too computationally expensive to solve them at scale. Today’s CAPTCHAs are basically Hashcash proof-of-work [0], but with the added benefit to Google et al. (and annoyance to users) that they help train computer vision models.