You're right! I forgot about those. A colleague and I tried to complete it independently but literally could not. One run would take multiple minutes and on the second try I was more diligent (taking even longer) and certain I did all the math correctly, but registration was still being rejected. Our new colleague did not sign up for GitHub that day and got the repository from a colleague who already had access instead

Edit: seems that's yet another one. Arkose <https://www.arkoselabs.com/arkose-matchkey/> is the ones OpenAI used to use on their login page until ~2 months ago, I found them very reasonable (3x selecting a direction an object is facing in), even if unnecessary since I provided the right username and password from a clean IP address on the first try