Use of c89 in GNU software

Question

In GNU coding standard it is said that free software developer should use C89 because C99 is not widespread yet.

1999 Standard C is not widespread yet, so please do not require its features in programs.

Reference here.

Are they talking about developers knowledge of C99, or about compilers supporting it?

Also, is this statement plausible as of today or is it somewhat "obsolete" or at least obsolescent.