@jwang11 message_filters is certainly not a simple case for migration. The location in ros_comm with the various other packages makes it more challenging to maintain a ros2 branch. A separation of that repository into smaller repo is certainly desired but would require a significant amount of effort.
Currently a separate branch or fork is required since we can’t support both ROS versions from a single code base yet. In the (near) future the goal is to make that possible though. As soon as that is the case I would prefer to merge any ROS 2 specific patches directly into the default branch and aim to minimize conditional code as much as possible.