This topic maybe a bit controversial but while researching Kyusho's history we can not overlook any possible sources. The term Kyusho is a Japanese derivation from the Chinese version of Dim Mak or Dim Hsueh., however as I have alluded to in many past posts or discussion videos, there are far older sources.
These sources all involve the pre-planned process and training of attacking Vital Targets. In all Waring Nations, Cultures, Tribes and Times, attacking Vital Targets was the main goal or what they trained to accomplish in battle. When in battle, you can't always get that clean Strike, Stab or Slash into a Vital Target, we all understand this concept as did those in that way of life and combat. But even if you missed a Vital Target by two three four inches, you're still wounding,disabling or killing the opponent.
In the olden days they didn't resort to hand to hand combat on the battlefield (unless last resort and mostly fruitless against an armed opponent). as they used Spears, Arrows, Swords, Lances and a number of different sharpened instruments, (Such as Daggers, a favorite of Hohan Soken) that was their designated battle weapon or a favored weapon.
As we look back into the old scrolls, books and notes, such as Jujitsu manual as, Fujita's records, Hohan Soken's compiled notes, even the Bubishi and older documents, they were written centuries or less ago. But there documentation that dates back to thousands of years and that is the Talmud, the Torah and the Bible.