What aerodynamic improvements allow variable geometry wings to be omitted on modern fighter aircraft?
It is more a tactical rather than an aerodynamic improvement.

Wing sweep is helpful when the top speed of the aircraft is well above Mach 2 and should be maintained for more than a few seconds. However, studies of past conflicts have shown that this speed capability was never used and almost never helpful even in hypothetical scenarios.

In addition, the lighter and stiffer structures made possible by carbon composite technology lower the maximum sustained temperature those structures can endure. Now the maximum Mach number is not only limited by aerodynamics, but also the stagnation point heating. In case of the F-22, the maximum sustained Mach number was lowered from 1.8 to 1.6 during development.