These plates are generally issued in Panini products like National Treasures and Plates & Patches. My recommendation to get started is go to to COMC and search (including Sold Out cards in the filter) the set the card made from the plate is from such as "2020 Panini Legacy" plus the word "plate". If any have existed on COMC, it'll autofill some suggestions that are probably what you're looking for, so you might see something like "2020 Panini Plates & Patches - 2020 Panini Legacy Printing Plates Yellow".
That lets you confirm where the checklists should be (in the above example it would be 2020 Plates & Patches). Then your next step is to do your due dilligence to try to find out which plates were inserted. Was it just cards 1-100 but none of the rookies? The whole set? The entire insert?
Once you have that info you can enter missing set inaccuracy reports under the products (like Plates & Patches), include the info, and have someone add them for you.
I hope that helps!