So there is some validity to the horizon staying somewhat stationary in certain situations. Not necessarily in a Flight Sim, but in a racing game in VR, if the head of the player is rigidly attached to the movement of the car, the up and downs of the road will make it VERY uncomfortable to drive. In racing games, horizon tends to stay stationary because the amplitude of the up and down and to some extend roll motion on a rough road could be very unpleasant to the player.
In a flight sim, since the motion is much smoother, even in tight turn combat in such games as War Thunder you don’t experience the same level of discomfort. Also it wouldn’t make sense for the aircraft to be rolling around you, as your eyes stay level with horizon. In a racing game, with the horizon staying stationary it sometimes feels like the car is pitching and rolling around you, but since the motion is very very limited only to a few degrees even at the steepest of the camber angles, its not as distracting.
I remember when they turned off the horizon motion compensation in Dirt Rally in one of the patches, it was impossible to drive until they got it back in.