Formerly, the rule was simple: introduce new features in a new distro, so existing users are not impacted.
With Indigo LTS, I found myself wanting to support new hardware that was not available when Indigo was initially released. An example is the popular Velodyne VLP-16 LIDAR.
Since I believe I can add that model with minimal disruption to existing users, I’ve decided to make an exception to the old rule, which made more sense when there was a new ROS distro every six months.