Strong nilpotency conjecture for Goursat germs

A Goursat germ is a germ of a Goursat distribution, and a tangential germ is one belonging to the tangential class. Mormul's Goursat conjecture. Goursat germs are strongly nilpotent if and only if they are tangential. The source states that this conjecture remains unproved, even though Goursat flags have been studied for many decades.