Still, I am worried, since Google does not clearly communicate these facts, but when you look at their communications, you read only about "Google+ Signin" and you have to search to find that you do not need Google+ but just the new protocol OpenID connect.
I don't like to see such communications from a company, that once claimed (long, long time ago!! Do you remember Google?) "Don't be evil".
It is OK, when they move away from an old protocol, but it is not OK, when they (and it very much looks that way to me!) use it as vehicle to market their products. Google has troubled people enough with forcing G+ on them -- so I even find it more troublesome, how they do it now (as it seems to me currently).
I fear, Google is still hunting after Facebook and is by the way inheriting its bad habits.
As I see now, they also want that a new button is used -- something with G+ on it. I see, they really lay the pressure on the people -- many people will think, that they need a Google+ account to use this feature (even if not required). I would say, Google you are going to hurt yourself!
I myself, was thinking about using a "Login with Google" button in my application ... but now, with things changed, I will think twice, before I do such a move!