This is bit of hypothetical, but does some sort of pki-like scheme exist that would allow me to hold a certificate (of sorts) from an authority that I could use to prove myself to an service, but also simultaneously would not leak any information about me to that service? Similarly service a and service b should not be able to link accounts behind my back. Sounds like a interesting crypto problem
There'd need to be some central authority (like you said) doing the certificate issuing that verifies that you are, indeed, a human, and that your a unique human that they haven't already given a certificate to.
Chances are they'd want more than a phone number; probably a photo ID or something as well, else their value proposition isn't very strong.
I'm not sure if that's better for privacy and safety than many services asking for just a phone number (which I can generate a semi-throwaway Google Voice number for).
