It would remove far more than 10%, here in PA a lot of our hunters are what would be considered social hunters. They hunt as part of a group but it's more about time spent with the group rather than actually hunting. Many of the ones we had dropped out over the years but there are a few that still hunt. That type of test would immediately be a non-starter for them even if they could pass it as they don't want to take the extra time to do it.
Additionally on top of that who gets to dictate the test and how difficult would it be? It wouldn't be hard it incrementally increase the difficulty to make it extremely difficult to qualify and get a hunting license. And then does it need to be a yearly test? One for every license? One for all types of arms or one different one for rifle, bow, shotgun and muzzleloader? See how it could easily spiral if allowed?
I will admit I am a relatively poor offhand shot, or at least I think I am, so I brace myself for almost every shot I take at game regardless of how close it is. A moderately small offhand target with a time limit would wash out a substantially large portion of shooters possibly myself included.
When I did the only guided hunt I've been on the guide required we check the rifles prior to the hunt to ensure they didn't get bumped on the plane. This also had the dual purpose of showing how proficient the hunter handled their firearms. He said he has had clients previously that he had to show how to load their rifle just prior to the hunt.
I was apparently more worried about it than he was as he told me my first shot was fine but I wasn't happy and tweaked my zero shooting three more times with my main rifle to get it where I wanted it.