OpenID Connect, aka. 'OIDC', is widely deployed. It's a compatible extension on top of OAuth 2.0, the latter of which was often used to provide third-party apps some delegated access to users' data at some data silo. This all coincided with the proliferation of HTTP-native APIs inspired by the easiest-to-implement tenets of REST. The payload of OIDC and OAuth 2.0 can be carried with in HTTP headers, which is a good fit for these APIs and simplifies implementation.
OIDC is used by some branded 'sign-in' buttons like 'Sign-in with Google' [1], and the new 'Sign in with Apple' feature is a close copy of the OIDC, if not yet directly conformant and compatible [2].
