Knowledge of standards before C99 is not "essential" unless you're on a legacy codebase. Which, to be fair, embedded, is very plausible. But then you're looking for an expert candidate anyways.
There are actively-used platforms that do not have a fully C99-compliant compiler (unpleasant, I know, but such is life :-( ). Also, many products in the embedded field have very long lifetimes (10-15 years), during which they at least need to be maintained. There are a lot of actively-used platforms that did not have a C99-compliant compiler ten years ago, and they are not exactly legacy codebases.
Edit: oh -- and (this I find truly revolting) there are companies that have not updated their coding standards and mandate that all written code should target an older standard (usually C89).
