There's a new script available from @gurkesaft - which more closely represents Head Physics. It's great and has a few different settings you can tweak for driver preference. There is no lock to horizon vs car. You always move with the car and head physics are applied to stabilize the track/view.
AC Head Physics
INTRODUCTION Your head, neck, body, inner ear, brain, and eyes together provide phenomenal vibration isolation and image stabilization. This is the one thing I have felt was missing from Assetto since day 1, especially in VR. I was thrilled to...www.overtake.gg
EDIT: I messed something up on this version. Does not work correctly.
Adjusted so that glance left/right will work as well as right mouse look around. This has limitations, up/down rotation only works off angle. left/right works as intended.
For those on screen/drifters, added "Slide and Steering" following. This uses most of the logic from the original NeckFX script. Only applies when using screen, default is off.
Adding settings.ini for easier tweaking to your own tastes. Renamed a couple values and added hints/explanations into CSP interface.
Also adjusted my default values slightly after running with it a bit longer.
Added math to try compensate and better differentiate between fast bumps and elevation changes without the camera lagging behind too much.
Updated default/recommended settings as well as a demo video.
Thanks to @W.Fox for testing out screen use and letting me know about the lag.
Sorry about that - the script files should be inside it's own directory.