Back in April Intel enabled the NIR IR by default within their Mesa driver but initially only for fragment shaders. Intel has now enabled NIR usage by default for vertex shaders within their i965 DRI driver.

For Broadwell and newer, NIR is now being used by default as the intermediate representation in dealing with vertex shaders. Using NIR led to a reduction of a few percent in the overall number of instructions generated for their reference test cases. Up to now the INTEL_USE_NIR environment variable had to be enabled for doing NIR VS while now it can be set to false if you wish to disable the new code-paths.

Details on this change affecting vertex shaders for the Intel Mesa driver can be found via this Git commit.