I think this is just a matter of definition. Deceleration is nothing else as an acceleration with negative sign with respect to the direction of the motion. So a decelerated motion is an accelerated motion in the opposite direction of the motion and then is an accelerated motion.
If we accept this definition, any accelerated or decelerated motion is an accelerated motion. Then a non accelerated motion cannot be decelerated, it has to be with constant (or zero) speed.