I bet you cannot buy that part specifically...you probably have to buy the entire Door Panel Assembly.
I am sure this was a cost saving feature by GM to eek out a few extra bucks of profit...save $0.50/vehicle * 300k vehicles, is $150k right to the bottom line (don't get me started on the price of these rigs...but I see this everywhere in business. Engineers design a product, get it in production, gets turned over to "value engineering team" who works on getting more profit out of it!)
That Said, if we could get someone to pull the pads out of the 2021 and share the P/N form it, we might have a chance.
Otherwise, look for a totaled 2021 in a JunkYard/You Pull It auto parts, and go get it?