OpenGL 4.3 Support Lands In R600 Gallium3D Driver
The R600g driver is now able to officially expose OpenGL 4.3 support. But the big caveat is that's only for the R600g-using hardware exposing FP64 support right now... That means just the Radeon HD 5800 series and HD 6900 Cayman series... All the rest of the HD 5000/6000 series and other R600g-supported hardware is still limited to OpenGL 3.3 support.
Cayman GPUs are now among the select pre-GCN Radeon hardware having OpenGL 4.3 via the open-source Mesa driver stack...
There has been work on "soft" FP64 support for these other GPUs so they too could move on to seeing official OpenGL 4.x support, but there is nothing ready for merging yet. But with no major Linux games relying upon FP64 functionality, overriding the exposed OpenGL version tends to work as well for these other vintage Radeon GPUs to get some GL 4.0~4.2 (and now 4.3) games potentially working. The MESA_GL_VERSION_OVERRIDE and MESA_GLSL_VERSION_OVERRIDE environment variables are what's worthwhile if you are in that boat.
This OpenGL 4.3 support for R600g is in place after finishing up the robust buffer access code (ARB_robust_buffer_access_behavior) and some other bits in the early morning hours of today.
The Radeon HD 5000/6000 series hardware is capable of OpenGL 4.4 support while still blocking it in R600g Mesa is not supporting ARB_enhanced_layouts and ARB_query_buffer_object. We'll see if that support comes in 2018 or if it's effectively the end of the road for new R600g extensions. AMD hasn't been really working on R600g these days as they are busy enough with supporting the recent cards while David Airlie has been among the community contributors still improving these around a decade old GPUs.