The most egregious example is from the classic "Turkeys Away" episode, there was a whole joke about Pink Floyd's "Dogs" which Johnny Fever was playing at the time. In the edited version Mr. Carlson just walks out mid-conversation.
And everyone knows that answer will be: "nothing stops potential distributors from licensing music now and releasing originals, the fact they are not willing to pay is not copyright holders problem". And there's some truth to it, licensing costs back then were lower because they were time constrained. No one would pay more for broadly licensed music in throwaway series. No one knew they would have lasting impact on popculture.
Or, often, because the original production bought licenses imagining more limited uses than later distribution opportunities presented, and chose not to spend extra money for things they didn’t expect to need.
Is there a list of the differences somewhere? I remember that show having a very good soundtrack.