Thanks for @PeterKampf's answer and reference for the illumination. However, I'm not sure I agree with the conclusion drawn within its reference.

As shown in Anderson, Fundamentals of Aerodynamics, the Newtonian theory is an ok first cut at hypersonic aerodynamics: due to the vast difference between the speed of sound and the airspeed, air flow is basically deflected by whatever shape it encounters. For a flat plate at an angle of attack ($\alpha$) with the hypersonic airflow, the bottom surface (incident with the airflow) will have a coefficient of pressure:

$$C_{p,l}=2\sin^2\alpha$$

while the upper surface will have no effect:

$$C_{p,u}=0$$

Combined together, this makes a coefficient of lift:

$$C_l=sgn(\alpha)*2\sin^2\alpha\cos\alpha\approx sgn(\alpha)*2\alpha^2$$

Therefore, for a flat plate, the lift is nonlinear with respect to the flow incidence. For a vertical stabilizer, the directional stability would also be nonlinear.

With a wedge shape making a half angle $\theta$, and for flow incidence smaller than $\theta$, the lift is:

$$C_l=2\sin^2(\alpha+\theta)\cos(\alpha+\theta)-2\sin^2(\alpha-\theta)\cos(\alpha-\theta)\approx2\theta\alpha$$

Therefore, for flow incidence smaller than $\theta$, a wedge shape has a linear lift with respect to flow incidence.

I suspect this is the reason why the X-15 has a blunt trailing edge on its vertical stabilizer. Please comment if you see an error with my conclusion.

Thanks for @PeterKampf's answer and reference for the illumination. This is to complement the results drawn within its reference, which I don't think are completely accurate.

As shown in Anderson, Fundamentals of Aerodynamics, the Newtonian theory is an ok first cut at hypersonic aerodynamics: due to the vast difference between the speed of sound and the airspeed, air flow is basically deflected by whatever shape it encounters. For a flat plate at an angle of attack ($\alpha$) with the hypersonic airflow, the bottom surface (incident with the airflow) will have a coefficient of pressure:

$$C_{p,l}=2\sin^2\alpha$$

while the upper surface will have no effect:

$$C_{p,u}=0$$

Combined together, this makes a coefficient of lift:

$$C_l=sgn(\alpha)*2\sin^2\alpha\cos\alpha\approx sgn(\alpha)*2\alpha^2$$

Therefore, for a flat plate, the lift is nonlinear with respect to the flow incidence. For a vertical stabilizer, the directional stability would vanish at small perturbation and would also be nonlinear.

With a wedge shape making a half angle $\theta$, and for flow incidence smaller than $\theta$, the lift is:

$$C_l=2\sin^2(\alpha+\theta)\cos(\alpha+\theta)-2\sin^2(\alpha-\theta)\cos(\alpha-\theta)\approx2\theta\alpha$$

Therefore, for flow incidence smaller than $\theta$, a wedge shape has a linear lift with respect to flow incidence and a fairly constant stability derivative even at small perturbations.