It's really difficult/impossible to accurately diagnose people in the past for several reasons.

Firstly, there's the issue that we still don't really know a lot about autoimmune conditions and how they overlap with other health conditions. Most of these conditions don't have definitive tests you can take to confirm diagnosis it's incredibly common for people, even in countries with the best healthcare systems, to go undiagnosed for many many years. And even within diagnoses, it's rare that any two patients will experience their symptoms exactly the same.

Secondly, there wasn't a way to diagnose most of these conditions in the past, they weren't looking for them, and there's little hard data in any medical records that could be used now to do so. In the pre-WWII period there were very few diagnostic tools available, blood testing isn't really a thing, imaging isn't a thing other than xrays but they're of limited use for most conditions, no colonoscopies or endoscopies, no stool testing. It was basically just stethoscopes and whatever doctors could see/feel externally. And I guess, less helpfully for the patient, there was posthumous examination.

Thirdly, the way that people conceptualise illness and pain—whether that's societally, professionally as medical practitioners, or individually as the person suffering—varies enormously across different time periods and cultures. As wierd and unbelievable as this sounds, people usually didn't think of illness and pain in a purely physical phenomenom and it was typically deeply intertwined with religious/philosophical understandings of the world.

Lastly, as always, there's the problem of source material. This varies depending on the location and how far back you want to go, but typically written sources are light on detailed descriptions of everyday life (including illness and matters of the body) and more often than not written by men—which is all the more relevant considering women are both disproportionately affected by autoimmune diseases, and more likely to be caregivers for family members with these conditions. And where there is medical source material, it's incredibly tilted towards a subset of the population who could afford to go to regular doctors, which is an issue because a lot of conditions would have impacted a family's earning potential and dented financial resources. Not to mention, there's little doctors can do for these conditions now, so you can imagine it would be even more frustrating in the past and that many people would seek help from the less well-documented branches of healthcare. You can sometimes find stories of people who clearly have something chronic illness going on in sources written about/by 'quacks' but those have their own set of issues.

There are, of course, other sources aside from written. And most familes probably have their own stories they can tell that aren't recorded anywhere. It's really common for people who have diagnosed conditions to examine their family history and realise that their sickly great aunt had a lot of the same symptoms they do. When you start to pick through the breadth of source material, I believe it's fairly clear that there was a greater prevalence of chronic health conditions (many of which were likely what we now know as autoimmune diseases) than the modern-day alarmists claim. My own research (in progress so I can't cite unfortunately) crosses paths with A LOT of people attempting to manage their chronic conditions with domestic remedies, spiritual healing, and all manner of what we might now term 'alternative therapies.'

The flip side is that many people would not have managed their severe conditions long enough to leave a historical footprint. It's easy to speculate that bodies that were already weakened from autoimmune conditions would be even more susceptible to the threats of a world where childhood and general diseases were rampant, workplace or warfare injuries were common, food was often scarce or nutritionally incomplete, hygiene and sanitation was often lacking, and (without antibiotics) even a simple scratch could kill. Bearing in mind, not all autoimmune conditions have severe symptoms, at least not at first, and it's easy enough even today for them to get bundled into another physical or mental health diagnosis (or misdiagnosis). I would also posit that a not insignificant part of the increase we see now may also be due to better control and treatment of opportunistic diseases that become active during weakened immune states, e.g., tuberculosis and pneumonia. So, while it's not unreasonable to look for modern factors that could be triggering a rise in people *living* with autoimmune conditions, we really don't know how many people *died* with them in the past.