x11.conf is one of my first memories about anything Linux. I remember when I was just trying it (back when “edgy eft” was new) via live booting it, the very first customization I had to make was editing that for to allow my extra mouse buttons to work (the back/forward buttons that are common now were pretty new-ish back then, and by default it only worked with the 2 main buttons and the wheel (and the wheel button).