I found out that
ReshadeEffectShaderToggler
has capability of reading the projection matrix value, and therefore encoded there FOV value.
In this case, the value encodes horizontal FOV, and the menu option in the background represents vertical FOV. I used the following formula to verify the variable:
90°=2*atan(1/0.5625 * 9/16)
Now how to use that functionality to attach the variable to the shader? Any ideas? The game is
In Sound Mind
if interested.