Realistically, it was changing during the '80s as the "Hickman revolution", and a focus on playing through stories and more detailed characters took hold. I think it became the de facto assumption during the heavy trad '90s (although gamer communities are islands, especially pre-internet, and there have always been a ton of isolated enclaves), and the idea only started to get re-interrogated during the rise of the OSR in the early to mid '00s.
I still know plenty of players who are strongly opposed to any sort of "out-of-character" metagaming, simply because they aren't familiar with the concepts that we're discussing here that have become more prevalent over the past 15-20 years.