If you are interested in the history of this topic I'd recommend reading "Twice Dead: Organ Transplants and the Reinvention of Death" by Margaret Lock. She compares the receptiveness over the years of the idea of "brain death" and organ donation between the US and Japan, two economically similar counties with very different rates of organ donation.

She also details how the idea of "brain death" came about due to medical advances saving people from death, particularly breathing machines, as well as the need to pronounce these people legally and ethically dead in order to "donate" their critical organs such as their still beating heart.

In fact, "brain death" only became a legal term in Japan in 1997 with the Organ Transplant Law, barely making under this subreddit's cutoff.