Google Authenticator

Used for 2 factor authentication.
Google may stop it going forward.
But you can use other apps that use Time based OTP (TOTP) like DuoMobile or Authy.

Google is trying to push people towards Google Prompts, feature that is built into Gmail and allows you to receive auth approval prompts as a push messages on your mobile device but that’s whole different idea with using Google as identity provider.

MSFT does the same with Azure.