Just to frame things as someone alive at the time in one part of America… single serving glass bottles were not returned to the store, they had no deposit. They were thrown away. Beer cans had no deposit. They were thrown away. (Steel soda cans weren't a thing, they stayed in bottles until aluminum came around) Fast-food cups were waxed paper. Straws were paper. The lids were plastic.
Companies were up front about controlling littering, using their brand loyalty to achieve it. McDonalds even had a "we don't like to see our name thrown around like this" campaign showing a clearly branded paper bag in a ditch.
Most roadside litter was steel beer cans, glass bottles (many broken), paper wrappers and bags. Any picnic area would have beer can pull tabs all over the place. It was actually an effort to retrain people to put the tab into the can so it eventually got thrown away instead of just tossing it where the barefoot children played. Coors introduced a special beer top which had a big hole and a little hole to jab your fingers through so there wouldn't be a beer tab to deal with! Eventually the aluminum lid with no loose parts was developed, but that was a decade after the anti littering campaigns.
It's taken half a century, but now most people don't chuck garbage out their car windows. Some people do. Some areas of the country are much worse than others. If you think littering isn't a thing, then it probably isn't where you live. But, there are areas where the roadside ditches are still full of fresh litter.