Originally Posted by
ThermionicScott
They started half-filling the drive-side dropout when indexed shifting came on the scene (mid to late 80's). Indexed shifting works best when you always have the axle in a certain position with respect to the jockey wheel of the RD. Eventually, they just went to vertical dropouts as that completely removed any question of having the axle too far forward or back (or crooked.)
Ditto. Or, alternatively: index shifting required more precise axle location than "normal" horizontal dropouts offered, but manufacturers were gun-shy about the tolerances required to implement vertical dropouts. The half-filled horizontal dropout made for good index shifting while still offering some wiggle room on chainstay length.
Within a couple years, frame manufacturers had tightened up tolerances enough that vertical dropouts were practical and common.