UPDATE 08/03/15: Starting today, API v2 of comments, captions and video flagging services are turned down.
------------------------------------------------------------------------------------------------------------------------------------------------------UPDATE 06/03/15: Starting today, most YouTube Data API v2 calls will receive 410 Gone HTTP responses.
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UPDATE 05/06/15: Starting today, YouTube Data API v2 video feeds will only return the support video.
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With the recent additions of comments, captions, and RSS push notifications, the Data API v3 supports almost every feature scheduled to be migrated from the soon-to-be-turned-down Data API v2. The only remaining feature to be migrated is video flagging, which will launch in the coming days. The new API brings in many features from the latest version of YouTube, making sure your users are getting the best YouTube experience on any screen.
For a quick memory lane trip, in March 2014, we announced that the Data API v2 would be retired on April 20, 2015, and would be shut down soon thereafter. To help with your migration, we launched the migration guide in September 2014, and have also been giving you regular notices on v3 feature updates.