Reference books are how you find things out

What actually happened

The skills being taught were real skills, and they were labor. Finding whether a library held a book meant walking to a wooden cabinet and flipping through cards filed by a scheme you had to understand. Finding a magazine article meant the green Reader's Guide volumes and hoping the periodical was on the shelf. A single citation could take an afternoon.

The collapse was fast. OPAC terminals replaced catalogues through the 1990s, JSTOR arrived in 1995, Google in 1998, and Google Scholar in 2004. The Library of Congress ceased printing cards in 1997. OCLC printed its last catalogue card on 1 October 2015, which is a reasonable death date for the format.

What inverted is the scarce resource. The old curriculum optimized for retrieval, because finding information was the expensive step and evaluating it was mostly handled upstream by librarians and publishers. Now retrieval is free and evaluation is the expensive step: anyone can find ten thousand sources in a second, and the hard part is knowing which ones lie. Information literacy instruction has shifted accordingly, but if you graduated before roughly 2000, you were trained for the opposite problem.