So, I can't answer from experience, but there were some significant changes since 1950 regarding the use of national parks.
In the 1960's or so, there was a significant uptick in usage of National Forest primitive areas, on the order of a 3x increase. Recreation visits to National Park Service areas went from 33 million to 172 million over the span of 1950 to 1970. Additionally, visits to US Forest Service lands increased 20x from 1924-1999, from 4.6 million to 900 million. (source)
In summary, the number of people utilizing the outdoors drastically increased in the time period since 1950. As such, it doesn't even require people "throwing garbage around, cutting down trees, leaving fires unattended" to destroy things; things as simple as going off path, which as a single person doesn't change anything, but when that becomes hundreds of people, it adds up noticeably.
Additionally, it's not even unattended campfires, but any campfires at all that will leave noticeable impact. First is simply the number of people scouring the area for dead wood, but also, fires will leave a noticeable, lasting impact on the soil beneath them. I'd say that "leaving fires unattended" isn't just a "Leave No Trace" thing, but a general "don't burn down the forest" thing which people already knew. However, things related to minimizing campfire impact were less well known.
Here's some additional reading comparing past and current National Parks, relating to various aspects. You could also look to see if you can find a camping manual or three from the time period you're interested in, to see what the recommended practices were.