[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.

[pxc]

Oh! You're right. It was way back in high school, and I think I must have learned about Latin-1 under the guise of "ASCII".