D is generally a much better Go (especially if you add fibres to it), but it doesn't have Google-brand behind it, so Go ate it's lunch in distributed systems space, while Rust is much better suited to be a serious competition to C++.
All of D except the backend was open-sourced in 2011, I'm pretty sure. Even before that, Walter Bright accepted patch submissions.
why ?
The code generation for DMD has fallen a bit behind, mostly because I spend my time on other issues these days :-(
Where do people get this from? D compiles down to the same machine code that every other compiled language does and it has similar language semantics to C++. In some cases D is _faster_ because its richer type and effect systems allow for more opportunities for optimization.