Aside: Not exactly this, but Amazon, Google(signup page) and few other big infra providers still use squiggly textual captcha. ReCaptcha is used by small players(no offense intended) as ux is often not main priority. You can try to spam amazon/google et al. signup pages and you'll be greeted by those sqiggly of olden days. :)
I've seen stuff like this in use for domain specific knowledge for forums for years - either there is a near infinite question pool or you're reduced down to something that's broken easily given a site-specific bot.
Over 10 years ago, off the shelf forum spam software, Xrumer, would build a database of these questions for you when it detected the field on a /register page. Since basically every website has a small, finite pool of questions (usually just one), you'd just sit down, answer them all, and then click "resume".
