ARB_parallel_shader_compile provides a way for the game/application a hint so it can limit the number of threads to be used to compile shaders as well as a means of querying for when the shader compilation process is complete.
O'Callaghan commented on the patch series, "This is my early draft on the threaded shader compile "suggestion" thread. The requirements according the spec are rather weak as it is up to the implementation, which frankly can even just ignore the requested thread count. However, afaik in the case of radeonsi it is already threaded. Anyhoo I just wanted to get some feedback."
RadeonSI is the prime benefactor of this extension for now but doesn't appear to have too much benefit at this time.
ARB_parallel_shader_compile isn't currently part of a released version of the OpenGL specification.