Basically, any copyrighted material should do. This only covers not ablated LLMs though. Ultimately, a web of trust where someone whose humanity and integrity you trust AT THE COST OF YOUR OWN INTEGRITY vouches for an individual is likely going to be the most reliable way. New open source contributors will require proof of humanity.

On top of this, you would want an expirable proof of humanity to reduce account takeover as a security vector and publicly sharing failed proof attempts would quickly deter actors from targeting projects en masse.