I understood dropout design to be mostly driven by race needs. Track ends that open to the back and are still used on track bikes and now, many single speeds and non-velodrome fix gears have been around since the first chain driven safety bikes. Horizontal dropouts opening to the front came along to make wheel changes faster and easier, especially with the then brand new concept of derailleurs. (Note - derailleurs were driven by racing. Racers had been changing gears in mountainous races for probably two decades before the first derailleur but it was by stopping and flipping the wheel around to a different cog. Those bikes HAD to have a horizontal slot for the hub to set the chain "tension". (Tension in quotation marks because it is always the proper chain slack that is sought.) After the freewheel was adopted for racing, a chain tensioner (basically a derailleur that doesn't shift) could be used. Now, I have never learned whether European racing adopted freewheels before the derailleur that required them. I know racing on the road stayed fix gear in this country until the derailleur took over in the 30s (by which time US racing was basically dead).

The word I heard when I was racing in the mid-70s was that vertical dropouts had taken over in pro racing because wheel changes were faster. Indexing had nothing to do with it; in fact it was years before it even existing at the racing level. My 1979 Peter Mooney wold have been vertical dropped except I requested horizontal dropouts. So I could run fix gear if I ever chose. (For the first 38 years, I second guessed myself on that many times. - until I set the bike up fix gear. What a ride! Now is a road fix gear in the elegant English tradition with a second life as a triple crankset gravel bike.)

The discussion I see there is interesting but misses the driving timeline of pro racing. The vertical drop switch was driven there before Shimano was even a player in the pro road scene, never mind SIS.