Both Gen 1 Decora Smart® products with HomeKit Technology (DH1KD, DH6HD, DH15S) and Gen 2 Decora Smart Wi-Fi products can be learned into the Apple Home app even if you lose the manual or box with the QR code.
During the traditional Add Accessory process a QR code on the box or manual is scanned by the app. If you are learning a Gen 2 Decora Smart Wi-Fi product a QR code can be found in the lower right corner. Simply remove the wall plate, scan the code and replace the wall plate.