Not completely true, there was some kind of power saving starting in those days:

If I remember correctly, one of the first moves towards power-saving in the stock-OS was Win 95/98/ME (one of those) to start using the HLT instruction (stop processor up to next interrupt) in the main OS, was a busy loop until then.

Notebook manufacturers (I worked with machines from Toshiba and Compaq in the 90s...) provided DOS TSRs and windows applications and drivers to control the CPU speed and display brightness.