Marek Olšák, one of the most frequent and notable independent contributors to the Radeon Mesa stack, copied over the compiler into the R300g tree. Per his commit message, "r300g doesn't depend on files from r300c anymore, so r300c is now left to its own fate. BTW 'make test' can be invoked from the gallium/r300 directory to run some compiler unit tests."
Not many use the classic R300 driver any longer. The R300 Gallium3D driver has been the default driver of choice on R300 through R500 ASICs for two Mesa releases. This Gallium3D driver is faster, cleaner, and offers more opportunities than what is supported by its older sibling.
While the R300c driver rarely receives new work these days, it's a low maintenance burden so as such it still will likely be living within the Mesa tree for another couple of releases. Along the same lines, ATI/AMD hasn't been quick to kill off UMS (user-space mode-setting) support while their KMS (kernel mode-setting) support is now rock solid.