The constancy of the speed of light in a vacuum for all electromagnetic frequencies is a foundational principle of classical and modern physics, well established by definition and everyday observation.
The claim states that all frequencies of light travel at the exact same speed in a vacuum, which reflects the standard definition of the speed of light in special relativity and standard physics nomenclature. While high-energy astrophysical observations sometimes test for minuscule energy-dependent deviations predicted by quantum gravity theories (such as Lorentz invariance violation in paper [6]), the proposition itself is foundational physics and common knowledge as a baseline macroscopic principle.