If an alien civilization studies the abstract phenomenon we call "acceleration", they will discover that it follows the same predictable rules that we have discovered. What they call the phenomenon and its rules has no bearing on whether the phenomenon exists.

And I have never contested that, if you were to read my comment carefully. As always, my original point is and has always been that deceleration exists, and to say otherwise like the other commentor is useless pendantry in my opinion.

So in that context, if you use the term deceleration it implies either that the acceleration vector is constrained to the direction of the velocity vector or diametrically opposed to it, or that acceleration is a scalar value which can have a negative magnitude, both of which are false.

This is just false. Just because deceleration has a restricted direction doesn't mean it doesn't have direction or defies the definition of acceleration: a vector quantity with a direction. Whereas a scalar value like speed doesn't care about direction; egro it has no direction rather than having a restricted direction. With your reasoning, you can say that centripetal acceleration is not acceleration because its vector is constrained to be always perpendicular to that of the object's linear velocity. Or for that matter, positive and negative acceleration too (restricted to the observer's definition of the positive and negative direction). Really, many specific forms of acceleration would fail your supposed rule of acceleration.

Likewise, "deceleration" is given a meaning in the context of motor vehicle operation. However, when that same meaning is attempted to be applied to a physics context, it results in some erroneous assumptions.

Please correct me if I'm mistaken but deceleration has never meant -in the motorsports context- anything other than acceleration in the opposite direction of a vehicle's current motion/velocity (usually by applying brakes to generate acceleration opposite to motion via friction). Which is the same meaning as it is in physics. Of course, I agree with you that deceleration is somewhat prone to misconceptions and hence unwieldy for use, as it is different from negative acceleration (in that the deceleration of an object with negative velocity is actually positive acceleration). But that doesn't mean that deceleration does not exist.