Visuals become mandatory. This is the watershed moment for the index due to Michael Jackson's Thriller (still the best-selling pop album).
In the purest technical sense, an index is a list. For a librarian, it’s the back-of-the-book guide. For a web server, it’s the index.html file that shows you a list of folders. index of pop music
Now, let’s look at the 2020s folder. The list is messier. Visuals become mandatory