I think the flap you refer to dates back to when both leaded and unleaded gas was still available. To prevent inserting a larger fuel nozzle into a car requiring unleaded, these flaps or restrictor plates were added so that only unleaded nozzles, which were smaller, would fit. Since leaded gas has disappeared for so long now, newer cars lack this feature.
Older members may remember some people removed these restriction plates and filled their cars with leaded gas because it was cheaper and their engines often ran better. Only problem: the lead wrecked the catalytic converters. If you lived where your car needed to pass an emissions test, it became an expensive lesson.
History lesson over!!