I’m going to go out on a limb here and equate SVG support with D3 support, since (in my opinion) that’s the most useful part of the library.
With that in mind, this link should give you the exact browser versions that support it:
http://caniuse.com/svg
And this matches what you pasted from the D3 site: basically every major browser vendor except IE has had SVG support for many many versions.
Your question says “in practice” and that means SVG. Yes, I know there are some samples of using D3 with the non-SVG parts of the DOM, but the vast majority of the examples in the gallery are SVG-based.