The concept that you can distinguish one kind of entity (a human) from another kind of entity of similar intelligence, if they can both send and receive the same data, was always a myth.

Captchas worked well enough for a while, if you weren't visually impaired (or worse, deafblind), spoke English, could read (Latin) characters, and so on and so on, but they were always a hack. The hack is now failing.

The only sure-fire way to check for humanity is to ask somebody who tested it in-person, likely an ID-issuing government.