The GL_ARB_stencil_texturing allows texturing of the stencil component of a packed depth stencil texture, as outlined via the OpenGL.org registry. It was back in March when Intel implemented support for the extension within their classic Mesa driver.
This Sunday, Ilia Mirkin wired up the support for the Gallium3D drivers. In the case of ARB_stencil_texturing, it's an easy case of implementing the support within Gallium3D's Mesa state tracker, which then allows for "free" the Gallium3D drivers to have this support. No changes were required to any of the hardware drivers or other code outside of the Mesa state tracker, per this commit. Nouveau's NV50 and NVC0 drivers are marked off for having this OpenGL 4.3 extension support while on the Radeon side it's relevant to the R600 and RadeonSI drivers.