I looked at a few store bought plates, both switch and receptacle, single & 2-gang:
UL insignia on back of plate, (3 out of 4); plates were/are various species of "wood", pine, oak, TEAK, etc.
Back of the plate has a recessed metal plate the size of a 1 or 2 gang cutout box. I guess the metal plate satisfies UL's requirements. Metal plate appears to be < 1/16" thick. I'll have to buy one, and rip it apart.
I have seen these plates at the big orange, big blue, and a few specialty shops.
Will I accept them?? Installed properly; yes.