About compatibility issues,
If I wanted to play a modern beatmap in 2007/2008, I can't.In my imagination, the legacy export would produce the same map in an older osu file format.
I know it might straight up not be possible in the code (ex. storyboards), but even then, maybe you could just get as closest as possible with a stripped down version of it.
For 2007-2011, maybe also make it so bézier sliders don't get converted to catmull sliders, and if possible, fix the slight imperfection in the bézier sliders.