I wanted to close my Google+ account, but when I went to do so, one of the items in the “Read this before proceeding” section mentions:
- You won't be able to use the "Sign in with Google" button to log in to third-party apps.
I assume they mean OpenID, but I’m not sure.
Best Answer
I once closed or disabled my Google+ and next time I wanted to log in to Stackexchange I got a weird message about missing Google+ and it wanted me to activate it again.
So if you close your Google+ you will probably not be allowed to use OpenID until you activate it again.
At least that's what happened to me. This was a while ago so Google might have changed something since then.