I see them prevalent in fiction just as well. Looking at the first few pages of a few random works of fiction, continuing from my other comment: https://news.ycombinator.com/item?id=46400974

- The Great Gatsby, F. Scott Fitzgerald, 1925: 28 on the first 10 pages https://archive.org/details/greatgatsby0000fitz_i1g1/page/n9...

- Love among the chickens, P. G. Wodehouse, 1909: 15 on the first 16 pages (and some of them spaced and extra long; apparently this publisher had a very “inflationary” style!) https://archive.org/details/loveamongchicken00wodeuoft/page/...

- Of Human Bondage, W. Somerset Maugham, 1915: eight on the first ten pages https://archive.org/details/ofhumanbondage0000wsom_j3w4/page...

- Howards End, E.M. Forster, 1910: at least 49 in the first ten pages https://archive.org/details/howardsend0000fors_q9r3/page/n9/...

> some of them spaced and extra long; apparently this publisher had a very “inflationary” style!

It's pretty common to see a single em-dash for the comma-like parenthetical usage (p6 etc.) and a double em-dash for the "someone's dialogue was interrupted and cut off" usage (p15).

The "I'm redacting this name" usage (p11) often uses two em-dashes too, although Wodehouse('s typesetter) doesn't in this case.

You have successfully proved me wrong. I have read some of those books, and merely not noticed the prevelance of dashes! Perhaps that is proof they used them well?