After 50+ commits were pushed today, the ARB_internalformat_query2 OpenGL extension was enabled for the Intel Mesa driver. ARB_internalformat_query2 is required by OpenGL 4.3. The extension is described via the OpenGL.org registry for those wanting to know the details of this particular extension.
With that out of the way, there's just one extension left for the Intel Mesa driver for OpenGL 4.3: ARB_robust_buffer_access_behavior. I haven't seen any Mesa patches recently at least for ARB_robust_buffer_access_behavior. So once FP64 support is done for GL 4.0's ARB_gpu_shader_fp64 and ARB_vertex_attrib_64bit extensions, which as mentioned already should be soon, Intel just needs to complete ARB_robust_buffer_access_behavior before they should be in OpenGL 4.3 compliance.
While Intel has lagged behind RadeonSI and Nouveau in supporting OpenGL 4.0~4.1, Intel could quite easily become the first Mesa-based driver supporting OpenGL 4.3. The Nouveau and Radeon drivers have two extensions for OpenGL 4.2 needed (just one for NVC0) and then 5~6 extensions behind for OpenGL 4.3. Thus it's good chances of Intel's Mesa driver supporting OpenGL 4.3 by the summer release but it could be a tall order seeing NVC0 and RadeonSI get there for the next release in three months, but at least they may achieve the GL 4.2 milestone.