Its total velocity never reduces to zero until it lands, but at the highest point of its trajectory its vertical velocity momentarily becomes zero. Its horizontal velocity reduces due to air resistance from the moment it leaves the barrel so the overall shape of the trajectory is asymmetric with a longer, less-angled climb and a shorter, steeper descent.
For a parabolic trajectory, neglecting air resistance, consider the vertical and horizontal velocity components. The horizontal component will be constant throughout but the vertical component will change since the projectile is accelerating in the vertical plane. The vertical component will be zero at maximum height.
If you throw a stone from A to B, it represents the shell. Does it stop before it descends? If it were to stop, upward and forward motion would have ceased, so from that point it would simply fall vertically to the ground.