I read that article long time ago, and for me it's a hard disagree. A system as complex and quirky as Unicode can never be considered "plain", and even today it is common for many apps that something Unicode-related breaks. ASCII is still the only text system that will really work well everywhere, which I consider a must for calling something plain text.

And yes, ASCII means mostly limiting things to English but for many environments that's almost expected. I would even defend this not being a native English speaker myself.