Unless I’m misunderstanding you, I believe we are actually not breaking anything in any particular ROS release.
Nothing will be removed in Kinetic. It stays as it is.
ros-industrial/universal_robot has never been released into Melodic. No binary packages have ever been made available. There is only a melodic-devel branch which does not come with any guarantees of stability or backwards compatibility.
That is not to say we don’t try, but announcing these changes now, before the first actual release is exactly to avoid having to introduce breaking changes in an LTS.
ur_driver will remain available in Kinetic. It will not be released into Melodic.
If you need to use ur_driver on Melodic, you are most likely building it from sources in a/your workspace right now. You can continue to do that, even after it has been removed from melodic-devel. Just include a copy of the kinetic-devel version in your workspace.
Finally: ur_driver has not been updated since 2015 (almost 5 years ago now) and has never been compatible with anything but CB1 and CB2 controllers since it was written. It cannot be used with CB3 nor e-Series controllers. For those, the new ur_robot_driver should be used, and even with CB2 controllers you may want to look at ur_modern_driver instead.
And just to clarify (but you are probably aware): ur_driver != ur_modern_driver.
