There was a technical reason. This version removes websockets, which has performance improvements and solves a lot of outstanding issues.

Unfortunately the current captcha relies on websockets. The option is actually still there, it just causes an error on the registration page, stops registration from showing, and still doesn’t actually show a captcha.

Long story short, it’s just broken rather than deliberately removed (though originally they planned on dropping it which is why no effort went into solving the issues.