Finally, after waiting for a few months NVIDIA has released a new mainline driver which includes Transform Feedback support. Previously, you had to use their special Vulkan beta driver to get it.
The "VK_EXT_transform_feedback" extension is one that was made especially for helping support translation layers from other 3D APIs. In our case, it helps DXVK plus Wine (and so Valve's Steam Play) with certain Windows games when run on Linux.
Additionally, this 415.22 release also adds in the fixed-up support for Unity games not working and solves a build failure with the Linux Kernel 4.20 release candidates.
If you're on Ubuntu, keep an eye on this PPA where it should land in the next few days for easy installation and updating. I just hope the PPA doesn't break Vulkan again when updating like it did recently. I've sent the maintainers of that PPA a message to notify them of the new release.
For other distributions, you likely already know where to look. For those installing manually, see the NVIDIA website.
Hat tip to bird_or_cage.