Google announced developers to remove Google+ Sign-in buttons (which are used to access to services and applications) and replace with new Google Sign-in button. The new Google Sign-in button is a new graphic identity with a more minimalist design.

Company states that "We are announcing the migration of our product Google+ Sign-In for the new model. Make this change for your application is simple: just use the latest libraries with the default session configuration".

Here's a image that shows the new usage.