The slope of the scales has no bearing on whether percentages are meaningful here. The problem with both systems when it comes to percentages is that neither system has 0 set to a natural 0. This leads to an entirely arbitrary point on the scale where decreases in the unit will approach a 100% difference and then suddenly start decreasing again.

If anything Fahrenheit should be less insane because at least the artificial 0 is likely to stay much further away in the data they're quantifying so the percentages stay reasonable.

The slope of the Fahrenheit scale matches that of the Rankine scale.

I would still say that the in the Rankine scale percentage increases make sense, and Fahrenheit changes to not.

The thing that matters isn't the slope, but the zero point; "X% farther from absolute zero" is a useful measurement, "X% farther from an arbitrary zero point" is not. Especially when negative or zero temperatures are involved.

Ok. Then please explain what % does the temperature rise when going from 0 Celsius to 5 Celsius!

Early measurements were done by individuals and they were idiosyncratic to the process of discovery/calibration.

Kelvin is refined measurements used to relate to a wider scale of temperatures. Celsius is a metric human scale subset of Kelvin.