If you use any of the deprecated APIs and do not take action before April 20,2015 your app will not work.
What to do for authenticated APIs
All of the following Google Analytics APIs require a current version of the Google Account Authentication API:
If your app uses any of these Google Analytics APIs, you must upgrade to OAuth2.0. Visit our developer guides for details on how to migrate to OAuth2.0 and for more general information about OAuth 2.0. Be aware that after you upgrade, current users will be asked to reauthorize your app before they continue using it.
What to do for unauthenticated APIs
The Google Analytics Metadata API does not require authentication, but is still affected by this upcoming change. If you use the Metadata API, you must take the following actions:
1) Register your application in the Google Developer Console
2) Update your application to include an API key with each request it sends to the Google Analytics Analytics Metadata API.
For more information regarding this change, please read the 2012 Google Developers blog announcement of this change and the more recent 2015 reminder.
The Google Analytics Developer Relations Team