I suppose the extension you have in mind are the extra libraries for dealing with buffered io, unicode and concurrency. But I don't think those could be termed "PLAN 9 own standard of C".

To clarify, my idea of restricted was in the sense of a highly refined subset. Much akin to how one should use C++ for instance. A careful selection of the good parts in C. Essentially, I was paying a complement to Plan9. :)