I suppose one sensible way to handle this would be if a group of technical friends ran a distributed group of OpenLDAP servers with OAuth and SAML2 providers in

I suppose one sensible way to handle this would be if a group of technical friends ran a distributed group of OpenLDAP servers with OAuth and SAML2 providers in front of them. That way, if whomever was running the master could not maintain it any more, another friend could convert their replica to master and people opt out of the defunct auth provider. The collective group of friends could then host services that use the common authentication framework. Between OpenLDAP, OAuth and SAML2, that should cover most self hosted applications.