Google recommends to use their GoogleSignIn SDK for signing in users on iOS. Google has also deprecated support for embedded browsers. This is not an issue for devices running >= iOS 9 which have SFSafariViewController. However, on devices running < iOS 9, SFSafariViewController is not available, and the GoogleSignIn SDK defaults to presenting a webview for OAuth2 sign in, which shows the embedded browser deprecation message.
This happens with the current Sign in sample project in their repo as well.
It also appears to be the case that Apple does not like us pushing people out to Safari in order to sign in.
What is the recommended approach for Google OAuth2 Sign in on devices running iOS 8 and lower?
Google's official answer (found near the bottom of their blogpost) is that iOS 8 users will not be blocked from signing in via embedded browsers, but they will see a message telling them that they should upgrade their OS.