Too Long; Didn't Read
With Google Sign-In [going away], developers would need to work with new service called Google Identity. Google Identity service, Google has designed two flows: Sign-in flows - Apps which just need to know user’s email, name for auth. Authorisation flows - apps which need to access Docs API, Sheets API on behalf of the user. This has become a mess of docs and seemingly relevant code which will render your app unusable. So I am gonna make it simple for you to decide.