What Properties Might Light of Millennia Past Have that Today's Doesn't?

The scientific method is extrapolation with feedback. You extrapolate/hypothesize from what is known and you test the extrapolation/prediction to see if it matches what is known/observed.That's how we know what is constant and what is not.

Anything that we can't observe or gather data from is outside the sphere of science. You can't throw out science - e.g. evolution or Big Bang - based on something we can't observe or gather data from.

The Big Bang itself is based on observations. Anybody trying to understand the Big Bang must necessarily propose hypotheses and the testing of those hypotheses is an ongoing challenge for physics. What should we observe if the hypothesis is true?But you were talking about "many things that we can’t observe, or gather data from" and I said that they are outside the purview of science.