There's some explanation on this newsletter:
"Given the resources of Open Robotics, we have to make trade-offs in our development and maintenance efforts. Gazebo10, to be released in January 2019, will be the last major version of Gazebo based on the code in bitbucket.org/osrf/gazebo. A new version of Gazebo, based on the Ignition libraries, will be part of the Ignition Robotics release in February 2019. We will continue to support older versions of Gazebo including versions 7-10 through their planned EOL dates which reach out to 2023 for Gazebo9."