The GL_ARB_compatibility extension is for providing access to deprecated OpenGL functionality on versions 3.1 and newer of OpenGL. While this is nice for being able to access new and old functionality of OpenGL, it's a mess to implement and as such there aren't any plans for implementing the Khronos extension within Mesa's OpenGL.
In response to a question from Mesa creator Brian Paul, Ian Romanick wrote, "No. There are several 3.1 features that interact poorly with some compatibility features (especially TexBOs and some legacy texture formats). We'd have to jump through hoops to make some of these features at all, and I'd much rather have the time spent optimizing the features we do support, finishing OpenGL ES 3.0, and working on OpenGL 3.2 We'd also much rather spend time tuning Mesa and our driver for core features than debugging and testing compatibility features that few people are likely to use. You'll notice there there are essentially zero piglit tests for compatibility interactions. I sure don't want to tackle that project."
Eric Anholt of Intel echoed similar statements earlier this week during his Intel Mesa presentation from San Diego at the GStreamer Conference. GL_ARB_compatibility is basically a mess for driver developers so they'd much rather spend their time doing other things, especially considering they have a hard enough time trying to catch up to the latest Khronos specifications.
The proprietary AMD and NVIDIA graphics drivers do implement this compatibility extension on both Linux and Windows. The Intel Windows driver does provide GL_ARB_compatibility too. The only other major driver stack going without this compatibility support is Apple on OS X.