This was not set anywhere, code relies on MLOOPUV_VERTSEL everywhere.
BMLoopUV.select_edge was never returning True and wasnt updating uv
select state when set manually.
Afaict, there were no official Addons using this from python.
Resolves T65836
Maniphest Tasks: T65836
Differential Revision: https://developer.blender.org/D6772