The speed of light is VARIABLE but... establishing the truth would be suicidal:
"The whole of physics is predicated on the constancy of the speed of light...So we had to find ways to change the speed of light without wrecking the whole thing too much" https://motherboard.vice.com/en_us/article/8q87gk/light-speed-slowed
"Thus a constant speed of light is embedded in all of modern physics and to propose a varying speed of light (VSL) is worse than swearing! It is like proposing a language without vowels." http://www.thegreatdebate.org.uk/VSLRevPrnt.html
The speed of light is OBVIOUSLY variable:
Stationary light source; moving receiver: Loading Image...
Frequency measured by the source: f; by the moving receiver: f' > f.
Speed of pulses relative to the source: c = df (d is distance between pulses).
Speed of pulses relative to the moving receiver:
c' = df' > c
in violation of Einstein's relativity.
Any relevant experiment, if correctly interpreted, shows that the speed of light is VARIABLE:
John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE." http://philsci-archive.pitt.edu/1743/2/Norton.pdf