The reason it has been claimed for years that the speed of light is constant is because this idea supports the deep time required to make Big Bang and Evolution work. Without it it’s less proof that there is evidence for this. So science hid for the longest that they knew the speed of light was not constant and never was.
Reference: New Scientist
Physicists freeze the motion of light for one whole minute.
Speed of light slowed
Reference: BBC News
Speed of light no longer constant
Reference: Live Science
This makes light years wrong, the distant of stars and planets base on the speed of light wrong. Every theory and paper written using this idea that the speed of light is constant wrong.