[D900]

The D3x is usable until ISO1600, but then drops in IQ dramatically. You can use Software to denoise the ISO3200/6400 Files, but the result is far from good, compared to newer Sensors, like D4 or even D3s. ISO100 is very clean and accurate in color, but outperformed by newer cameras. But it is still a very fine workhorse, when using under very good and bright conditions.

In general terms, I agree. That's why I specified "(good light)" in what I wrote.  I am certain that in good light (let us be specific - at ISO 100) the D3x produces better images than can the D3s, D4, D4s or D5.

"usable" is subjective and also depends on the type of usee.  I'd go further than you; I found the D3x unusable at ISO 1600; even at ISO 800 it was sometimes unacceptable.

I don't know about the D800/800E/810, but I can say with certainty that the D850 is almost always usable up to ISO 3200, i.e. an extra 2 stops progress in 9 years of flagships (from D3x in late 2008 to the D850 in mid 2017).

And at ISO 100, the D8x0 series are the first Nikon dSLRs to clearly better the D3x.  The D6x0 and D750 were not better than the D3x at ISO 100; they were indistinguishable (in the case of the D600, assuming it had not spewed oil onto the sensor).