EDIT: Just realised the title is a bit misleading - to be clear, I just want to disable Y axis, vertical mouselook.

When it comes to the old games with auto-aim, I usually prefer to play without the vertical axis entirely (and in Build engine games, use keys to do that springy look up/down when needed). Just feels quicker and smoother to me.

But unless it's staring me in the face and I've missed it, I can't see any option for this in Fresh Supply; either you have mouselook with the warping, or you have mouselook with perspective fix. You can't even turn Y sensitivity down to 0; only down to 10%.

Is there any way to tweak a file somewhere to do this?

I'm bumping this. I would also like to play like this.

Considering that there is also no strafe modifyer key binding, ie. Shift+→, to strafe right, it may be problematic to play keyboard only.

As I said above - just looking to disable mouselook in the sense of "true" mouselook, Y axis movement. So that the game behaves the way it did back in olden times.

Is this still unsolved? The thing is I could have sworn there was a bind to press on the keyboard to disable and enable vertical aim

The "U" key was the default bind to disable/enable mouse aiming in most of the Build Engine shooters. You may be recollecting that.