Intel Lands Transform Feedback Support In Their Vulkan Driver For Mesa 19.0
VK_EXT_transform_feedback was added to the Vulkan spec a few months back for transform feedback support in Vulkan drivers for helping efforts like DXVK for running Direct3D 10/11 and other APIs on top of Vulkan. The VK_EXT_transform_feedback is needed for efficiently implementing Direct3D Stream-Out support and has been used by DXVK since then, VKD3D is also now using it now, and obviously for OpenGL-over-Vulkan efforts it could be used for providing GL transform feedback itself.
Intel's open-source driver team has provided VK_EXT_transform_feedback patches to their Mesa ANV driver since the extension was published, but it wasn't immediately merged over the lack of review and not having sufficient test cases for verifying the expected behavior. Well, that's all be squared away in time for Mesa 19.0 to help out Linux gamers.
At the end of several just-merged patches touching not only the ANV driver but also the NIR compiler and SPIR-V Mesa code, the extension is now enabled. Assuming your Steam Play / DXVK is up-to-date, you should then be good to go for utilizing this support with Intel graphics hardware.