This is simply not true. The concept has existed in common law since the Middle Ages, and has existed in the US since its inception. Of course the boundaries changed, for example the adoption of the Prudent Man Rule in the 1950s, ERISA in the 1970s, and the rise of litigation and regulation in the 80s and 90s. Pinning the current structure on one court decision is ignorant and misleading.The term is fiduciary duty. In the 80’s under Reagan the Supreme Court interpreted this to be the primary function of issuing a Corporate Charter.
It was stealthy, nobody noticed, but that was when things officially changed in America.