There’s good glutamate and bad glutamate. Everyone needs the glutamic acid (a.k.a. glutamate) that the human body has been creating in carefully controlled amounts since time began. It’s a building block of protein. It’s essential to life itself.
But manufactured free glutamate is a different thing. It was invented in 1957, at which time mass production of manufactured free glutamate began. And from that time forward, manufactured free glutamate has been easily available in foods, drinks, supplements and drugs, in the uncontrolled amounts that cause brain damage.
Amino acids did not cause brain damage before 1957.