The FBX SDK documentation says that normals should be IndexToDirect
mapped. 9 years ago, we didn't export with IndexToDirect mode because
apps at the time didn't support it well.
9 years is a long time and the apps I've tried all work with
IndexToDirect, so the export of normals has been updated to use the
IndexToDirect mode, using the already existing, but disabled code.
The FBX importer was not previously set up to handle the import of
ByVertice-IndexToDirect normals, so this has been added to ensure that
FBX IO exported .fbx files can also be imported.
