i think space-em-dash-space was pretty common in both journals and monographs on engineering, science, and math in the 01940s to the 01970s, including some that include really excellent typesetting, though i don't have an excerpt ready to hand to post (and hn isn't really a usable medium for discussing visual things anyway)

i don't remember ever seeing it in fiction

i think of space-en-dash-space as just being an error, and i'm pretty sure i wasn't just misidentifying en dashes with spaces around them as em dashes