Normally, we’d freeze the API/ABI a short time after the original release (June or so) - but since this is our last ROS release, we don’t have another release to push things off to. There are a number of PRs opened that break API/ABI - and that we should probably get in.
With the MoveIt project, we’ve decided that API/ABI breaking changes are allowed through at least the end of the year - to give developers time to get those changes in. I’m proposing the same for Navigation. If you have strong thoughts on the matter, please comment on the GitHub issue: https://github.com/ros-planning/navigation/issues/1038