If the original bat has a legal safety knob, and this doesn't change the characteristics of that knob, this isn't altering the bat; adding the cushion to minimize the sting doesn't change the physical structure. It is the same as adding a second layer tape to the grip, which is legal, as long as it still leaves the flare.
If this is added to a bat which doesn't have a legal safety knob, or is used to change the shape of the knob so that it no longer complies (flare or cone grip), then it is an altered bat.
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