Pandoc has had "smart" typography[0] which generates em-dash and en-dash for a long time. I found a forum post for 2011 where people were discussing em dashes and such. That thread indicates that John Gruber created a Markdown extension in 2004 which was already handling en and em dashes[2].

[0] https://pandoc.org/demo/example33/7.1-typography.html

[1] https://pandoc-discuss.narkive.com/PHmQaAgM/en-dashes-vs-em-...

[2] https://daringfireball.net/projects/smartypants/