If nobody used em-dashes, they wouldn’t have featured heavily in the training set for LLMs. It is used somewhat rarely (so e people use it a lot, others not at all) in informal digital prose, but that’s not the same as being entirely unused generally.

Microsoft Word automatically converts dashes to em dashes as soon as you hit space at the end of the next word after the dash.

I do—all the time. Why not?

I also use en dashes when referring to number ranges, e.g., 1–9

Except for Emily Dickenson, who is an outlier and should not be counted.

Seriously, she used dashes all the time. Here is a direct copy and paste of the first two stanzas of her poem "Because I count not stop for Death" from the first source I found, https://www.poetryfoundation.org/poems/47652/because-i-could...

  Because I could not stop for Death –
  He kindly stopped for me –
  The Carriage held but just Ourselves –
  And Immortality.

  We slowly drove – He knew no haste
  And I had put away
  My labor and my leisure too,
  For His Civility –

Except for highly literate people, and people who care about typography.

Think about it— the robots didn’t invent the em-dash. They’re copying it from somewhere.

Tell me you never worked with LaTeX and an university style guide without telling me you never worked with LaTeX and an university style guide.