The easiest design trick to make a website more reader friendly is to move a black font to a grey font. (Obviously there are exceptions and personal preference involved, such as yours)
Is there actual scientific research that supports this?
Most printed books have dark black ink instead of grey ink. (Although the paper of the pages may not be #FFF white but an off white.)
In the office, the laser printers lay down dark black toner on crisp white paper. The paper brightness can be ISO 98 or even ISO 102 and the black toner is extremely comfortable to read.
If your statement was true, I would think there would be a massive demand for dark grey toner printed on light grey paper because it is "less strain on the eyes."
The evolution of Amazon Kindles are also moving towards better contrast. Early models were dark grey e-ink on light grey background. The latest models are getting closer to the ideal of black text on white background and the customer reviews support that improvement.
A comparison datapoint: Using F12 inspector, I notice that sarahwerner.net paragraphs uses rgb(68,68,68) while HN posts' text use rgb(0,0,0). I think most of would find HN text to be more comfortable to read.
I don't know about OP, but in painting and in video/film art pure black and white are encouraged to be avoided. There they are called superblack and superwhite. In paintings it's because there is rarely (or not at all) such color in nature and in video (same for painting) also because it used to degrade signal for some reason in analogue era (strong yellow was avoided as well).
Apparently, it looks like there's a difference of opinions (rather than scientific rigor) whether the "dark grey" of #000 is "light enough".
I'm fine with it ultimately boiling down to subjective aesthetics but sgk284's post made it sound like #000 eyestrain was scientific fact.
We also try to avoid staring into the sun, for the same reason.
Polarization might be different, but that's something that's only barely perceptible to humans. I don't think it's plausible that it could affect legibility or eye strain. ( https://en.wikipedia.org/wiki/Haidinger%27s_brush )
There's a big difference between absorbing reflected ambient light and having photons pumped into your retina.
If people are going to tweak contrast they should have dark text on an off-white background, rather than strog white background and mid-grey text.