Sure they do, by definition code that uses the extensions is not standards compliant. Plan 9 C code makes heavy use of Plan 9 extensions. The Go runtime also makes use of those extensions, that's why gcc recently (2009) implemented -fplan9-extensions in order for gccgo to compile the Go standard library.
> Note that both C++ and GNU C have similar extensions but under different semantics which don't make them any less ANSI compliant.
I think what you mean is that GNU C is a strict superset of ANSI C and you assume that Plan 9 C is the same. This assumption is wrong. Plan 9 C is not a superset of ANSI C, some ANSI C things are missing; e.g. const. That being said it's not that hard to compile C89 code with the Plan 9 C compilers.
> be careful around those documents. The assembler, compiler, parser and linker have seen over two decades worth of work since those were put ink to paper.
I should know since I use Plan 9 every day and I recently ported the Go toolchain to Solaris and gave a talk on these tools. The compiler document is very accurate, the only glaring anachronism is that the extern register storage class doesn't necessarily use a register in the Go toolchain, but depends on the target operating system. It's still true on Plan 9 though. The assembly guide is more inaccurate, but the C papers are accurate, at least for now.