Even considering the underreporting with young, high school athletes, a large number of young football players in the US have suffered concussions[1] at a very young age when their brains are developing, and that's with equipment that we can reasonably assume is designed with safety in mind. If a nefarious government were to try to promote a sport, which had rules and equipment that lead to a higher risk of brain injury, and cultivated an image/environment that elevates "toughness"(not complaining when you feel you've been injured; a significant fraction (~16%) of high school football players continue to play even after being hit hard enough to lose consciousness, so that attitude is there, just need to bump the number up), that might get you close.

[1]https://en.wikipedia.org/wiki/Concussions_in_American_footba...