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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UPDATE: With the launch of video abuse reporting and video search for developer, the Data API v3 supports every feature scheduled to be migrated from the soon-to-be-turned-down Data API v2.
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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.

Retirement plan
If you’re still using the Data API v2, today we’ll start showing a video at the top of your users’ video feeds that will notify them of how they might be affected. Apart from that, your apps will work as usual.
In early May, Data API v2 video calls will start returning only the warning video introduced on April 20. Users will not be able to view other videos on apps that use the v2 API video calls. See youtube.com/devicesupport for affected devices.

By late May, v2 API calls except for comments and captions will receive 410 Gone HTTP responses. You can test your application’s reaction to this response by pointing the application at eol.gdata.youtube.com instead of gdata.youtube.com. While you should migrate your app as soon as possible, these features will work in the Data API v2 until the end of July 2015 to avoid any outages.

How you can migrate
Check out the frequently asked questions and migration guide for the most up-to-date instructions on how to update specific features to use the Data API v3. The guide now lists all of the Data API v2 functionality that is being deprecated and won't be offered in the Data API v3. It also includes updated instructions for a few newly migrated features, like comments, captions, and video flagging.

- Ibrahim Ulukaya, and the YouTube for Developers team