Medical knowledge of CTE is less than 20 years old, and documented evidence of brain trauma in boxers (for example) only dates back to the late 1920's. What piques my interest is how much people further in the past knew about TBIs, especially in people who would have intuitively been at greater risk for blows to the head (military personnel, athletes, etc.) Further, if any knowledge was available about TBIs, how were victims treated? Could they qualify for welfare for the disabled, for example?
As always, it really depends on which premodern societies you mean!
The Edwin Smith papyrus dates from about 1300 BCE, and is thought to be a copy of an older text, possibly 3000 BCE. It's a surgical treatise that discusses the presentation and management of various wounds, including injuries of the head and serious brain injuries. The descriptions of the injuries and the resulting pathophysiology are quite recognizable to the modern physician. The descriptions also show an understanding of some of the significant neuroanatomy (earliest known mention of the meninges covering the brain) and the effects of different injuries (such as the difference in paralysis caused by a brain injury versus that caused by a spinal cord injury, there's also a mention of the patient with a wound in the temple being rendered speechless) and the relative prognoses of different injuries (all the injuries are classified as "an injury I will treat," "an injury with which I will contend," or "an injury I will not treat ").
It's quite possible you'd be better off getting knocked on the head during the reign of Pharoah Djoser than in the court of Alfred of Wessex three and half thousand years later!