The idea that chess has not changed in a long time is simply not true. Two huge and relatively recent changes were the addition of chess clocks and premoves.
And aside from the mechanics of how the game is played, there have been massive changes in the popularity of chess (first massively upwards, recently possibly down slightly), as well as how analyses are done.
It would be very difficult to account for these factors in a way that keeps comparisons across 30-year+ time spans meaningful.
