> Nuclear winter is now understood to be either less severe than predicted back in the 60s,

Back in the 80s. In the 60s it was just megadeath, with a chance of mutants.

(The Krakatoa movie was in 1968, but the winter thing took a while to sink in)

https://en.wikipedia.org/wiki/Nuclear_winter <- look at "Early work".

But you are right that the concept was only made popular in the 80s, and a lot of the earlier works were classified or unknown and obscure to the public.