Wait, though: K&Rv2 is post-C89.
Not completely: it's published in 1988, but the cover says "based on
draft-proposed ANSI C".
Naturally K&Rv2 documents this, but if you want to know about
traditional practice the relevant wording should come from K&Rv1,
not v2.
I don't know what K&Rv1 says on the subject,
I think it's silent on the subject, but that's really old memory on my part.
Remember that this was in an era before most people were used to thinking
seriously about language semantics and thing that we now call "implementation
defined" and "undefined" weren't thought about.