Starting today, all public AMIs will have the deprecation time set to two years after their creation date. This means that public AMIs that are more than two years old will by default be deprecated. Once an AMI is deprecated, it will no longer appear in DescribeImages API calls for users that aren’t the owner of the AMI. Deprecating an AMI only reduces the visibility of the AMI in untargeted searches, but continues to be usable and available to you. Users of a deprecated AMI can continue to launch instances and describe the deprecated AMI using its ID.
Source:: Amazon AWS