I must say, having switched from Google OpenID 2.0 to Google OAuth2.0 for authentication on our site I raised an eyebrow when I belatedly read on Homakov's 2013 blog[1] that OAuth2.0 was never intended for authentication, only authorization for certain discrete resources. At least with Stack Exchange doing it I'm in good company, but - if Egor is correct - it raises the question what's really a good choice for authentication nowadays.
[1] http://homakov.blogspot.com/2013/03/oauth1-oauth2-oauth.html