[My1]

Couldn't one use u2f as a captcha alternative, obviously without information about the stick itself, only the batch attestation, and then throwing the registration in the bucket? After all it does need an interaction in the meatspace and sure a bot could be engineered to trigger it, but you can't just relay the challenge somewhere and have someone else clear it for you and even if you have a lego construction or whatever to clear your captcha, it's FAR slower than having many people on a solving service help you.