Federated logins are a thing! The challenge is finding one that’s open and privacy-friendly. Unfortunately the widest-used ones come from entities like Google or Facebook with a marked interest in preying on user data. Mozilla used to maintain a federated system (Persona) but they discontinued it. I know Ubuntu offers one for all their services (bug trackers, forums etc.) but not sure if it’s open to third party systems. Perhaps there are others worth using.