As you may have heard, Google+ is going to die. This is a good thing since there were some security / privacy issues last year. Google decided to kill the product instead of trying to fix it, which is understandable and a respectful decision.
Still, at some point Google+ was used as an authentication provider for several services. Google was kind enough to send us a message saying that Google+ is discontinued, and according to this message:
If you sign in to sites and apps using the Google+ Sign-in button, these buttons will stop working in the coming weeks but in some cases may be replaced by a Google Sign-in button. You'll still be able to sign in with your Google Account wherever you see Google Sign-in buttons
Notice that "in some cases may be replaced"? That means that you may lose access to your profile, and when Google+ is decommissioned, you may not even be able to know which services have such a profile for you. That is a privacy risk that you should not be willing to take.
If you don't want that to happen, you actually need to act fast. Especially since the only way to figure out who uses Google+ for your sign in, is to use the Google+ mobile app.
The instructions, provided by Google are straight forward. Still, since they rely on your access to your Google+ account AND the Google+ mobile application, I suggest you act fast.