The debian/copyright file does not very specifically document the
package. Please fix it in general. I have found this to be lacking
documentation for the files in Fonts directory, some of which I know to
be DFSG-free. Most prominently, monoMMM_5.ttf is probably not under a
free license. Other non-free candidates are 7-Segment.ttf, LED-8.ttf,
and LED-16.ttf.

I do not know about the debian package specific parts, but any issues
regarding the core FGData package should be raised upstream at
https://gitlab.com/flightgear/fgdata. Otherwise things get messy fast if
changes get made only here, and they will not benefit the whole community
which debian apt users are only a very small minority of.  There is a very
low chance that the people who can resolve this will be looking at debian
apt issues. I just stumbled across this by chance looking for something
unrelated.