As a general rule: I see no benefit to anybody in allowing long
Message-Ids, weird characters via escape mechanisms in them, or
anything similar. To the extent that they are generated and used only
by software, it doesn't matter what's there, and simpler is better.
To the extent that they are used by people, simpler is better even
more so.
Therefore, I support requirements that the Message-Id be restricted
for the ease and efficiency of software (implementation).
If anybody has a reason that a Message-Id should be able to contain a
Russian novel (in Russian), I'd be very interested in seeing it.
Headers (and body) meant to be read, understood, and entered by people
are, of course, a different matter.
Seth