Why do modern aircraft tend to have angular tails?
The rounded or elliptical design was found to be the cleanest aerodynamically in non-compressible subsonic airflow, which is why it's very common on aircraft of that era.  While the design is very slippery for speeds of Mach &lt; 0.6 or so, it is more difficult to manufacture, which is why, near the end of the war, Spitfires had the 'clipped' wingtips as opposed to the original pure elliptical planforms.

The modern low aspect, sweptback, trapezoidal tail design is a better choice for higher airspeeds and supersonic flight ie Mach > 0.8 and mitigating flight control flutter due to compressibility effects, the same reasons we use sweptback wing.  Note as well that modern fighters use the same kind of planform for their wings and tail planes as they offer a compromise between acceptable supersonic aerodynamics and good trans sonic maneuverability.