Problem is we cannot compare them since the tests change over time. My dad had to learn how to use a slide rule in school. By the time he started his first job though, the first electronic calculators were affordable and so he never used for anything real. Anyone just a couple years older than him would have needed it. By the time I got into school nobody knew anything about them, and so we cannot compare.
I'm sure completing the square has been an important algebra question for that last few centuries and so we could potentially compare how students do at this over the years. However the only thing we can ask is how many got it 100% perfect - if I got it wrong but I did most of the steps right how many points do I get - this is expected to vary by teacher. Even then though, it isn't clear if over the years education hasn't change - should we spend more time teaching this important technique at the expense of something else. Also not everybody takes algebra, and I wouldn't be surprised if scores change as we push more/less kids who naturally wouldn't do well in algebra to take it anyway.
I would hope you can think about the above and find a dozen more confounding factors that I haven't thought it.
Last I heard we don't even know how to measure that in the first place. As my intro to psychology class said IQ tests are a reliable measure, but what they measure nobody knows. (which is to say if you take the test more than once you will score about the same every time)