Why sloped armour can still matter against descending shells

Does plunging fire make sloped armour counterproductive?

No. Sloped armour usually increases the effective path through the plate unless the shell arrives at a closely matching angle.

Descending shell trajectories do not make sloped armour automatically counterproductive. Inclination usually forces the projectile to travel through a longer effective thickness of material. Only an impact arriving close to the plate's own angle removes much of that advantage. This is a geometry lesson rather than a promise of immunity because the exact trajectory and plate arrangement still matter.