Recently Mailchimp announced a new version of its API changing the version and thus making it more robust, this means that version 2.0 of the API will be deprecated. If you are currently using the 8.x and 7.x-4.x branches of the Maichimp 7.x-2.x and 7.x-3.x module there is nothing to fear, you are already using version 3.0 of the API , the story is different for those who are still using the 7.x-2.x and 7.x-3.x branches, those branches use version 2 of the API, according to Mailchimp that version of the API will work until December 31, 2016 so it's best to update.
Module: https://www.drupal.org/project/mailchimp
Mailchimp API v 3.0: http://developer.mailchimp.com/documentation/mailchimp/