WordPress.org

Scott Taylor

Atom Publishing Protocol support for WordPress is still supported in this plugin. 
Support for Atom was dropped in WordPress core in version 3.5.

Atom Publishing Protocol

Atom Publishing Protocol support for WordPress

Matthias Pfefferle

ActivityStreams (<a href="http://www.activitystrea.ms" rel="nofollow ugc">activitystrea.ms</a>) support for your WordPress-blog
ActivityStreams is an open format specification for activity stream protocols, which are used to syndicate activities taken in social web applications and services, similar to those in Facebook’s Newsfeed, FriendFeed, the Movable Type Action Streams plugin, etc.

ActivityStream extension

ActivityStrea.ms feeds for WordPress (Atom and JSON(-LD))

Atom Publishing Protocol vs ActivityStream extension