Why wouldn't it be? You have a hunting license to harvest animals in season.
I don't think there are any regulations that say you can't keep bones, skulls , antlers etc from non-harvested game.
Just my .02 but I've also never been in that situation
Call a GW. They should know the answer.
It says out of season. Also may depend on what causes the natural death. Are we talkin bout a 7 yr old or a 4 yr old? You could probably keep the rack but not the head if the animal needs to be tested.