Comment by czx4f4bd

I think they're referring to the old reCAPTCHA v1 approach.

From https://en.wikipedia.org/wiki/ReCAPTCHA:

> The original iteration of the service was a mass collaboration platform designed for the digitization of books, particularly those that were too illegible to be scanned by computers. The verification prompts utilized pairs of words from scanned pages, with one known word used as a control for verification, and the second used to crowdsource the reading of an uncertain word.