Hi Stephen,
The simulation team at Open Robotics is currently working on the release of Gazebo 11, which is planned to be the last version released from the osrf/gazebo codebase. As you mentioned, Gazebo 11 will EOL in 2025 (which, by the way, is also the expected EOL year for ROS Noetic).
The team is also working on the next generation of Gazebo, called Ignition. Ignition has already had 2 named releases (acropolis and blueprint) and the next one, codenamed citadel is coming up before the end of the year. Citadel will be Ignition’s first 5-year LTS, but we still expect large changes in the upcoming versions, as the codebase is currently under fast-paced development. You asked about competitions, and in fact, the SubT competition is already using Ignition Blueprint with ROS Melodic.
Having said all that, both Gazebo and Ignition are projects independent of ROS or ROS 2. Users are able to use ROS with other simulators, as well as Gazebo / Ignition with other frameworks. However, since Open Robotics maintains all of them, we make sure that they’re all well integrated 
Here are some more resources for you:
Cheers