As you can see from the "External Identities Initiative" issues tag, OpenID has apparently been removed from the core for Drupal 8 while OAuth is being added. However, the exact status is not clear to me because of the spaghetti-web of related issues, comments, modules, etc. at Drupal.org. There are a lot of Single-Sign-On (SSO) modules, including those in a comparative SSO table (that does not appear to be all-inclusive). Unfortunately, when people have been posting about SSO functionality, they aren't always clear about whether they are trying to enable Drupal to be a identity provider or consumer.
What I personally want to do is allow users of my Drupal site to log into that Drupal site by using their Google account credentials (so that Drupal is an identity consumer). I don't much care whether that is done via OpenID or OAuth. I have been unable to get OpenID to work. (I have been trying to simply add an OpenID to an existing user account and it does not work nor does it even give me an error message. For the OpenID, I have been using the Google account user's Google Plus Profile URL as is suggested at http://openid.net/get-an-openid/.)
I am going to try the OAuth Connector next for my use case. Hopefully it will work.
Comments
HostGator mod_security was my problem
I finally stumbled upon http://www.gerixsoft.com/blog/drupal/openid-page-not-found and similar pages and determined that HostGator's deployment of mod_security was my problem. After a chat with their technical support they did something that eliminated the 403 forbidden problem I had been encountering upon Google's post-authentication redirection back to my Drupal site. The index.php patch suggested at the aforementioned page didn't work for me.
OpenID removed from Drupal 8 core
This is unfortunate. Steam uses OpenID exclusively as far as I am aware. Does this mean I have to program my own module?