This is a really good point and I've added a question to my section to allow us to ask the question "what kinds of actions can be put under the umbrella of 'parry' and 'riposte'?". I think that you're thinking a little too specifically since while yes, shield moves are an excellent example of the concept in action, there's more parry-like actions that can be made in a match beyond simply lowering a foe's stats imo.As a Smash player I love this concept, it's a cool idea. If I think about a move like Marth's Down B, i feel like the shielding moves make a lot of sense: King's Shield, Baneful Bunker, Silk Trap, Spiky Shield protect the user from damage while inflicting some effect on the opponent if they use a contact move. Unfortunately, because of switching and how priority/speed can affect turn order it's hard to achieve a satisfying true parry unless you use taunt before the opponent tries to use a status move. Based on the definition you provided, it doesn't feel like there are enough viable game mechanics that allow you to prevent the opponent from attacking or setting up.
I mentioned it in the write up, but switching in a Pokemon with Magic Bounce when a foe attempts to set up hazards or use a status move is a parry in my eyes since you're active avoiding the negative effect your opponent is attempting to put onto your side of the field and returning it back on their end. Another concrete example I can point to right now is Venomicon-P that can switch in on a predicted resisted move and immediately get a +1 to its defense and a potential Nasty Plot to +2 SpA in the opening it has created, which immediately applies pressure on the opponent. another small example would be to switch in something with Rough Skin or Iron Barbs on a low-HP opponent attempting to use Rapid Spin, for example.
I suppose you could boil the concept down even further to "this Pokemon rewards good predictions and opponent reads through its inherent qualities" and it's up to us, should it be selected, to figure out what those abilities are.