It won't be affected. Google spent the last few years decoupling every useful G+ feature into standalone services, the account feature to me looks completely separate nowadays. Besides, without the OAuth provider, tons of integrations (that Google actively want on their products) would break.

G+ was such a silly play, when you consider that Google already had the key to centralized identity all along: the ubiquitous GMail account. They will continue to push that for sure.

I second that... basically all tutorials out there make use of the google+ api to log in.. would be a huge fallout all over the web ^^

This is why I gave up on oauth. In theory it's nice have somebody else manage your user accounts, but even facebook, which is not in imminent risk of a shutdown changes its API response periodically.