As far as we know, the speed of light as measured in vacuum has always been constant. We have not come across any experimental evidence otherwise.

Furthermore, we assume that all physical laws are the same across time and space. This is important due to Noether’s theorem, which says that symmetries in the physical laws lead to conservation laws. In this case, if the laws of physics changed with time, then energy conservation would be false; you would need very, very good evidence to claim that energy conservation is false.

As a caveat, changing the speed of light on its own isn’t very meaningful, because it’s a constant with dimensions. In physics, you can reframe all the most fundamental formulas in terms of dimensionless constants, like the fine structure constant; really, it’s these that would you want to see have changed over time or not.

For instance, if the fine structure constant changed with time, then the type of light emitted from atomic transitions would change over time as well. Meaningfully changing the speed of light would affect lots of other seemingly unrelated physics like this.