Not my field, but I do know at least one element of this: elimination of mass plays. In the early days it was less common to throw the ball: long forward passes caught by a running back and run across the goal line weren't allowed. Far more common was to run the ball down the field. A typical way to do this was for the offense to have squads in motion behind the scrimmage line before the ball was put in play, linking arms and forming a Flying Wedge: the quarterback would put the ball into play in the middle of the moving squad, and they would charge down the field, bringing maximum combined force against whatever seemed the weakest point of defense.

Rules were changed 1905-1906 to make that impossible: seven linemen had to form up on the line, and there were restrictions on movements behind the line. Long forward passes were allowed. It cut the death rate significantly.

Here you can see some diagrams and commentary on mass plays laid out by Alonzo Stagg in his Practical Treatise on American Football of 1893. The Internet Archive has the full text. Hard to imagine any defensive tackle risking his neck to block them....