I agree with you that Unicode is too complicated and messy, although it also shows that whether or not something is considered "plain" is itself too difficult.
Unicode has caused many problems (although it was common for m17n and i18n to be not working well before Unicode either). One problem is making some programs no longer 8-bit clean.
Unicode might be considered in two ways: (1) Unicode is an approximation of multiple other character sets, (2) All character sets are an encoding of a subset of Unicode. At best, if Unicode is used at all, it should be used as (1) (as a last resort), but it is too common for Unicode to be used as (2) (as a first resort), which is not good in my opinion.
(I mostly avoid Unicode in my software, although it is also often the case (and, in many (but not all) programs, should be the case) that it only cares about ASCII but does not prevent you from using any other character encodings that are compatible with ASCII.)
> ASCII is still the only text system that will really work well everywhere, which I consider a must for calling something plain text.
Yes, it does work well (almost) everywhere.
Supersets of ASCII are also common, including UTF-8, and PC character set, ISO 2022 (if ASCII is the initially selected G0 set, which it is in the ASN.1 Graphic string and General string types, as well in most terminal emulators), EUC-JP, etc. In these cases, ASCII will also usually work well.
However, as another comment mentions, and I agree with them, that if you mean "ASCII" then it is what you should say, rather than "plain text" which does not tell you what the character encoding is. That other comment says:
> Plain text is text intended to be interpreted as bytes that map simply to characters.
However, it is not always so clear and simple what "characters" is, depending on the character sets and what language you are writing. And then, there are also control characters, to be considered, so it is again not quite so "plain".
> 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.
In my opinion, it depends on the context and usage. One character set (regardless of which one it is) cannot be suitable for all purposes. However, for many purposes, ASCII is suitable (including C source code; you might put l10n in a separate file).
You should have proper m17n (in the contexts where it is appropriate, which is not necessarily all files), but Unicode is not a good way to do it.