"Back in the day" this wasn't even a consideration. It's only been very very recently that people have started thinking about plastics breaking down and going into the water system.
Obviously it was a concern since the very first plastic item was manufactured. But who was aware this was a concern? It's only been in the last 5 years or so that I can recall hearing anything about it. And only in the last 2 years or so has it really risen to the level of concern for me. Maybe we are reading totally different sources that it has been on your radar for decades.
As a datapoint... Back in elementary school 25 years ago, they most certainly were teaching my class that plastic doesn’t decompose and how important it was to reduce waste.
As one anecdata point, I was born in 1978 and I can't remember ever not having this concern. Not sure what you mean by very recently, though--one could say plastic was even invented very recently.
My parents grew up in an environment (semi-rural) where/when it seems to have been normal to leave trash around, though. This was in the 50s. I guess they saw the issues and raised my siblings and me differently.
It makes me happy that more people are thinking about not trashing their environment!
