Currently the speed of light is constant by definition of the meter. If we were to find certain cases where the speed of light appears to be different from c, that would be interpreted as compression or expansion of spacetime. For example, universal expansion can be reinterpreted as light being faster in the past.
I'm wondering whether it could be reinterpreted as time having a variable rate.
we only measure the two directional speed of light (bounce timing). The Einstein convention assumes that speed is the same in all directions. i'm not saying it's not. Just that it's impossible to tell otherwise. Einstein mentioned this in his paper.