Concerning headaches and eye strain caused by EVF I've read it in some forums where users reported that. Also two friends of mine have experienced that.
Concerning possible damage of the retina and possible cataracks and macular degeneration I've first heard of it last year in a science TV channel. They said there are first serious hints to that, but further research is needed.
Concerning the problem of degeneration / short-sightedness caused by too short viewing distances (e.g. smartphone use) you'll find numerous resources by a Google search. Depending on the key words (and language) you use (I've got 200,000 - 280,000 hits depending on the search words).
I've used Sony, Fuji, Olympus mirroless cameras in the past a lot.
But unfortunately with all of them I've got headaches and eye strain when I've used them more intensively / for longer periods.
That was one of the reasons why I stopped using mirrorless and came back to SLRs.
I wondered whether it was just a personal problem of me. Therefore I've asked my eye doctor. He is a very passionate photographer for decades, too. And he explained to me that EVFs in general have disadvantages concerning the eyes. He explained it to me in an extremely detailed way but as I am "an analphabete in medcine" I don't understand all.
But he was very clear in his results and recommendations:
1. From a health point of view OVFs are much much better than EVFs.
2. EVF can indeed cause headaches and eye strain.
3. With intensive use there is the risk of getting short sigthed and getting macular degeneration.
4. For professionals and enthusiast amateurs which use their cameras a lot he recommends OVFs to avoid eye problems.
5. The eye degeneration problems by the short viewing distance of smartphones are meanwhile very well documented.