You are basically correct in your first statement , The vast majority of guns were made like this .
There may have been guns like live pigeon guns that were built as target guns but these in my experience have been few and far between .
You bought a gun and learnt to shoot it . No one bothered with all this point of aim / point of impact stuff and it seems generally confined to enthusiasts on forums as with talk of theoretical ballistic effects , that gunmakers of the late 19th and early 20th century did not concern themselves with ,if indeed they even thought about .

In tests with lasers mounted centrally in bores and sighting down the rib at a target 25 yards away , it was found that on average the right barrel shot high and to the left of the center and vice versa . The actual cross over point some 12/15 feet from the muzzle .These tests did not take into any ballistic affect of the cartridge .
The tests were carried out using barrels form a variety of British and Continental guns of all grades and values .I can not speak for guns made in the USA .

With O/U's the general rule was that the top barrel shot flat and the bottom up to 12" high [ this was later prove using solid slugs at a target]
These tests were carried out with the idea that when sleeving a pair of barrels the alignments would be achieved more accurately to achieve the theoretical ideal configuration .

Like many others in the trade we had an excepted belief that the average 12 bore with 28" barrels would have over lapping patterns at 25/30 yards , so you can imagine the results came as quite a shock .

You can work out mathematically the theoretical dimensions that a pair of barrels would need to achieve the desired results .Take the convergence of centers ,from breech to muzzle , than project that convergence to a 25yard point .
Taking into consideration the centers at he the breech can be 1.1" to 1.1/4" work out the centers at the muzzle on a set of 30" barrels and see what muzzle centers they would need . Also work out with the same centers for differing barrel lengths as any maker using the same action would do and you will begin to see the points I have tried to get across .
I have never seen any gun that will meet the requirements and thats about all I can say on the matter .