Looking for research regarding pure black vs dark gray for readability

I've heard people say that websites with pure black text on a white background can cause exhaustion when reading, and that it's better to use a very dark gray text or a light gray background color to decrease the harshness of the contrast.

Does anyone know of any research that was done to support this?

I'm scouring the internet, but not finding anything that was done recently even though you can look at many prominent content sites and see plenty of examples of this in practice.