file updated with 1893 and 1907 - I'm done now, I swear
Here's a line graph depicting the ITPHR percentages over time according to the data for the seasons I did. The big spike was in 1901 when the American League arrived, and the big temporary drop in 1915 was due to the Federal League having a low rate. The rate declined sharply for good at the beginning of the lively ball era and probably dropped to a little under 1% in the mid-1950s or so. Today in MLB there are usually about 15 inside-the-parkers hit per season, for an even lower rate of 0.3 or 0.4 percent, one would think due to smaller parks and the disappearance of Astroturf. More to come.