If we want to take a biological or evolutionary viewpoint it gets rather interesting. Autism would be fine and even beneficial, but in it's most severe forms it would be very detrimental to survival and the passage of genes.

However, we will never know how that presented in the past. Our modern environment almost certainly influences the presentation and manifestation of what we classify as "mental illness".

I would say any behavior, at least from a biological standpoint, that does not impact an individual's ability to survive and reproduce would be "neurotypical".

Of course human culture and socities make this very very difficult to interpret. Especially considering that our ability to survive in the modern day ia directly related to our ability to do labor. Labor that is well outside of the behaviors we evolved for.