I took a quick look at the D8 code, since I’m going to have to deal with converting before too long. Oauth2_server 2.0 does define /oauth2/UserInfo as an endpoint in oauth2_server.routing.yml, so you may have a configuration issue.
It took me forever to get everything working, by the way, so I can sympathize. In case it helps, here are some of the settings in my R.C custom oauth setup.
Token path: /oauth2/token
Identity path: /oauth2/UserInfo
Authorize path: /oauth2/authorize
Scope: openid email basic offline_access profile