[astahlx]

I started using emdashes in my academic career, after my advisor pointed me to the subtle differences. And since then, I like and use emdash a lot. In Latex, it is easily produced, just keep the spacing rules in mind. The Punctuation Guide is a nice reference on it https://www.thepunctuationguide.com/

[globular-toast]

There are actually four different "dashes" in La/TeX. The hyphen (-), en-dash (--) which is used for numeric rangen like 1--2, the em-dash (---) for punctuation, and the minus sign ($-$). Knuth talks about them in the TeXbook which is good fun.

[pxc]

I think you can do all of those in plain text as well. There are Unicode characters for those dashes and probably more

[globular-toast]

Not in ASCII. My definition of plain text is roughly "the characters I have on my keyboard". Unicode is like a superset of all possible plain texts. Useful, but I really don't like my own files containing characters I can't (easily) type. If I regularly typed in another language I would acquire a keyboard for that language. I'm not even convinced typographical symbols like various dash types even belong in Unicode at all to be honest. It seems like you have to draw a very arbitrary line somewhere.

[Symbiote]

Drawing the line at "OK-ish for American English" is far too restrictive.

You can't write CO₂ or m², use a fraction like ½, claim © or mention a price in Euros or Pounds Sterling.

You can't even write major American place names (San José, Oʻahu).

[globular-toast]

It's not too restrictive for me. I rarely need to write foreign place names or words (I'm British). Yeah I use the £ symbol so I'm not limiting myself to ASCII, just what is on my keyboard (I have € too). I just don't really consider a file full of characters I can't type to be "plain text" just because it's UTF-8, that's all.

[pxc]

I'm pretty sure © and ½ are in ASCII. I think é might be, too.

But anyway, I agree: there's no reason plain text shouldn't be rich.

[JdeBP]

Wherever you learned ASCII from, it was very wrong. It probably made the common (although less common in the 21st century than in the 20th) erroneous conflation of ASCII and Latin-1, or IBM code page 437, or IBM code page 850.