Sounds like Cloudflare Privacy Pass, invented in part to allow Tor users to move around without being stuck in captcha hell [0], I heard about them from someone who wanted to sell anonymous access to a service by handing out these tokens that can be redeemed but not tied back to the point of distribution, but I've never actually seen it implemented this way.

  In short, the extension receives blindly signed ‘passes’ for each authentication and these passes can be used to bypass future challenge solutions using an anonymous redemption procedure. For example, Privacy Pass is supported by Cloudflare to enable users to redeem passes instead of having to solve CAPTCHAs to visit Cloudflare-protected websites.

[0] https://privacypass.github.io/