Honestly - the best “policy” here for things outside the very core ROS is probably just to document what you’re doing. For released packages, index.ros.org will tell you which branch should work for which distro (although, I’m not sure the beginner docs really highlight how to use index.ros.org).
Putting a note about branching conventions near the top of the README of the git repo can also be beneficial. I used to do this all the time back in the ROS 1 days when we did things like indigo-devel works for indigo & kinetic, then we branched to melodic-devel for melodic and noetic, and hey there is an experimental ros2 branch too.