It never was.
But C, the speed of light, was postulated to be a constant by Einstein. Now it’s looking more like a decaying function.
Bad idea, disrespecting einstein... you are wrong BTW...
I have long wondered if the redshift were actually caused by decay of photons (the speed of the particle constant, but the wavelength decreasing). If so, the universe may not be expanding at all. If neither the speed nor the wavelength of light is constant, what do we really know?
Also interesting is ‘knot theory’ physics...