There is no one view, but the estimates of prevalence going back decades suggest that there must be literally hundreds of millions of undiagnosed people. That is not "rates increasing exponentially", it is that we are just about starting to catch up and recognise the extent of the issue.
A good analogy is left-handedness. Has it increased? Probably not, but we don't beat children for using their left hand any more. Or homosexuality went through a period where it looked like it was increasing very fast and "everyone" was coming out, but actually it's just safer and more accepted these days (as it should be). Despite moral panics, a majority of people remain heterosexual and likely always will.
While there may have been people living in previous generations with ADHD who got by and managed, I reckon there were probably just as many or more who were less visible because they were shunned from or found it difficult to access mainstream society. You used to have borstals for children whose behaviour was not quashed by physical punishment in schools. Dunce caps and basically no effort made to understand why children might find it harder to learn than others. The terms "Educationally sub-normal" or "ineducable". It wasn't unusual to leave school without qualifications, in fact it was relatively recently that you could perfectly legally leave school before even taking a single exam. Of course I do also recognise that for a lot of people with ADHD who might struggle with academics, this could actually be preferable particularly at a time that qualifications weren't necessarily always needed for every job. But there will have also been people who left school with nothing and struggled to access much of "normal" society because of this.
Workhouses going back further, where conditions were terrible and disease ran rampant. Child labour (mines, factories) with high accident rates. Unmarried mothers were sent away and their children adopted. There have been criminal underworlds, drug/mental health problems, and fringes of society, always. Travelling circuses full of "freaks" (obviously not ADHD related, but there are similarities to what I'm saying - people born with physical differences, who can now live in society normally because people understand and/or they can have things corrected surgically or hormonally) The anti-authoritarian hippy movement of the 1960s, punks, anarchists, and other subcultures were probably all quite accepting of people with ADHD.
I think if you did a detailed study on this somehow you'd probably find that people with ADHD often struggled to access various aspects of "normal" society, and were less visible because of this. Now that we have more modern ideas that children must be protected from being forced into work, that education is worthwhile for all children, that notions about "deserving vs undeserving poor" are simplistic and don't tell the whole story, that addiction is a disease and mental health issues do not mean a person is evil, it makes a group of people who were often overlooked by the values of the past more visible and so it seems like they have increased, but I think they were always there, it's just that a lot of them were in prisons and slums and borstals and communes and squats and other non-standard ways of living. Or they died early.
Something else which might be interesting to look at this way is SIDS / cot death. Until the 1950s, infant mortality in general was so high that it didn't really register as a thing - babies were just fragile and could easily succumb to illness or suffocation. Only throughout the 50s and 60s when infant mortality had reduced enough from other causes did these unexplained deaths start to stand out and pose a question. There are obviously references to babies dying during sleep in earlier historical periods, so it has probably always existed, but if you look at official figures, you see rates climbing from the 60s through the 70s and 80s (England and Wales peak was 1988). It seems quite unlikely to me that the number of incidences was actually increasing during these decades, more so that the recognition of it as a phenomenon was getting better. Then safe sleep advice during the 80s and 90s started to bring numbers down successfully, which is why we have lower numbers (and lots of advice) today.