Every ROS Distribution has a set of targeted platforms. You can see the list here: https://www.ros.org/reps/rep-0003.html
Melodic specifically targeted Stretch. It is not unexpected for things to compile from source on more platforms than are officially supported if they are close enough. Users can choose to take advantage of this for their own specific purposes. However we don’t plan to change targeted platforms during the lifespan of a ROS distro.
It turns out that doing a forward port of all the packages is approximately the same amount of effort as creating a whole new ROS Distro. But it has the drawback that the maintainers have to go in and change things and actually makes it less stable for the users on the originally supported platforms while patches to support the new platform are added.
So the direct answer to your question is that there are no plans to make a new version of Melodic targeting Debian 10 Buster.
However in good news for you. I mentioned that it’s just as much work to make a new release. And along those lines we are starting to prepare for the next ROS release, Noetic, and it’s targeting Buster. The PR for the REP is here:
It’s very early in the pre-release cycle but we have bootstrapped up to over 100 packages already in our testing repository The release is targeted for World Turtle Day in May.