I tested it with Bowser (B-Stick) and Snake in training mode, and from what I have seen, the Dspecial is cancelled by ledges that can't be grabbed. To do this, you land the Dspecial so that it hits at the end of the space where Bowser can stand without wobbling on that ledge, and a character shields the attack (doesn't have to be a power shield). I replicated this on all neutral stages and some banned ones, with the exception of Smashville (it's hard to do it by yourself on the moving platform). Very situational, but it does good shield damage with low risk of punishment afterward.