What is an abstraction barrier?

Structure and Interpretation of Computer Programs talked about abstraction barriers as a way to hide the intricacies of data structures made out of cons cells. Is this concept still useful in a world of literal hashmaps with string keys? I say, yes, but in a much more limited way than before. In this episode, we go into what abstraction barriers are, why (and why not) to use them, and the limits of their usefulness.