If it had existed for thousands of years, we would already have a name for it and probably some treatments. So the conclusion is that this is a new disease that did not exist before.
Why that is not valid, illustration number 2,
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1142241/
Possibly the earliest documentation of multiple sclerosis is the case of Lidwina the Virgin, who lived in Schiedam, Holland. In 1395, age 16 years, Lidwina developed an acute illness and subsequently fell while skating on a frozen canal. Later symptoms included blindness in one eye, weakness and pain. She died in 1433.
Then almost 500 years later,
The features of multiple sclerosis were first well defined by Jean-Martin Charcot, neurologist at the Hôpital de Salpétrière in 1868, as 'la sclérose en plaques'. In particular he made the distinction between the tremor of paralysis agitans (later called Parkinson's disease) and that of multiple sclerosis. The three most reliable indicators of multiple sclerosis—intention tremor, nystagmus, and scanning speech—became known as Charcot's triad.
Edit:

Yeah, I will tell about it.
Another family thing involving MS here shows that a medical thing long before a diagnosis for it, even a name for the condition exists.
Aunt Susan was diagnosed with MS around the time she was in 7th grade.
That diagnosis stood until a few years before the pandemic when she did one of those family ancestry genes things.
She'd been a nurse while her health allowed, and upon getting the ancestry gene results, went, "That's kinda interesting."
Took it to her neurologist at KU Med center, who went, "It's even more than kind of interesting, how'd you like to make a road trip to the Mayo clinic?"
Mayo Clinic did some tests and came back with, "You do not have MS and have never had MS, what you do have is really bad luck ... this pairing of ethnic genes we now know don't play well together, this second pairing of ethnic genes which we now know don't play well together, and for the hat trick, this third pairing of ethnic genes we now know don't play well together."
I seriously doubt Aunt Susan is among the first humans to have health problems as a result of ethnic genetics.
Who can say how far back in human history that condition goes.
I would not be astonished if it goes well back before we knew there even were genes.

The principle here is,
when humanity and health providers do not yet know in detail what a health thing is, it will get misdiagnosed as something else.
So, no telling how far back ME/CFS may go, and we at this point on the calendar can only surmise what things it might have been previously misdiagnosed as.