It is deprecated, but we’ve avoided removing it to prevent unnecessary disruption between releases.
I summarized a bit of the history in this GitHub comment:
https://github.com/ros2/common_interfaces/issues/1#issuecomment-112621348
Ultimately, it was decided that we would remove it in ROS 2:
https://github.com/ros2/common_interfaces/pull/2
However, I’m not certain whether or not we’ll ever do so in ROS 1.