Can you name some books as example? I picked a few random books from the period you mentioned, both fiction and non-fiction, and checked the first few pages of each; most of them had a good number of em dashes (or spaced en dashes, depending on the publisher's typographic style). For example:- Leave It to Psmith, P. G. Wodehouse, 1923: five on the first two pages https://archive.org/details/bwb_O8-BSS-318/page/10/mode/2up
- Kim, Rudyard Kipling, 1901 (1913): fourteen on the first three pages https://archive.org/details/dli.pahar.1530/page/1/mode/2up
- The Sun Also Rises, Ernest Hemingway, 1926 (1954 printing?): ok, I admit Hemingway was very spare with punctuation; I noticed none https://archive.org/details/bwb_C0-BHF-057/page/2/mode/2up
- Men of Mathematics vol 2, E. T. Bell, 1937 (1953 printing): two in the three pages of the Preface https://archive.org/details/MenOfMathematics/page/n5/mode/2u...
- The Story of Philosophy, Will Durant, 1926 (1962 printing?): seven in the first five pages https://archive.org/details/THESTORYOFPHILOSOPHY1TheLivesAnd...
- The American Language, H. L. Mencken, 1919: ten in the four pages of the preface https://archive.org/details/americanlanguage00mencuoft/page/...
(The counts are just the ones I noticed; there may be more.)
Are the books you read very different, or do you have a different threshold for "rare"/"niche"?