Pre-industrial would mean that a lot of energy use would be the in-efficient use of wood burning. I recall that part of J. S. Bach's payment at one of his posts was firewood provided by his employer -- something like "8 cords of birch wood, 2 cords of Alder, split for kitchen use.... I think the list was about 15 cords of wood for a year. Bach had a big household, but even so.
Certainly the use of coal at the start of the industrial age clouded up the air substantially. See records of London 'pea soup' fogs. Coal was used on farms too, but the spread out nature of farms would dilute it somewhat.
I live in a rural area where many people heat with wood in winter, and some with coal. At this point with an average of one household per square mile it makes little impact. 6 miles away is a coal fired power station. On a really bad day I can just barely see it's plume. The dust collection is pretty good.
In pre-industrial times, farms averaged about 5 acres per family unit, about 100 times the current Alberta farm density, and 1000 times Saskatchewan's farm density.
Ok: What are the contributors to visibility reduction:
Plant transpiration -- water vapour is a big one, but plants also put significant amounts of complex organics into the air. Most of the 'Blue Ridge" mountain haze is the result of plant activity. This will change depending on ground cover. Deciduous trees are more prone to this than grasses and forbs.
Dust from natural activities. Only occurs in climates dry enough that ground cover is incomplete.
Man made dust. Tillage in agriculture is the main cause of this, but watch any gravel road too.
Smoke from combustion. Much depends on how stuff is burned. Large particulates (e.g. diesel soot) fall out of the air column fairly quickly.
My wild ass guess is that average visibility hit a minimum somewhere between 1930 and 1970 when tillage and coal use was at it's peak, but has since increased due to both less coal being burned, and better combustion design, along with increasing use of zero or minimal tillage in agriculture.
You could get one check on this by looking at particulate measures in glacial ice.