So after wanting to tear my hair out for quite some time trying to find an acceptable GUI solution, I randomly stumbled on this and have high hopes the search might be coming to an end.

Unfortunately, I ran into a bit of a progression stopper: I can't find the D3D9 integration sample. It's not in the samples and I can't find an older version to download to check if it was there. I know I can (probably) build my own integration, but I'd rather not reinvent the wheel if I don't have to.

I know D3D9 is ancient by now, but I currently don't have a choice to keep using it (porting to D3D11 would be a large job). Is there any sample/reference implementation available somewhere?

D3D9 has support in previous version NoesisGUI 1.2. I didn't checked if D3D9 is supported in 2.0 - the NoesisGUI team changed renderer architecture so perhaps they've dropped D3D9...
Our game is built with NoesisGUI 1.2 and using D3D9 when D3D11 is not available.

We don't provide a D3D9 reference implementation for now. We are still deciding if doing that or not. But now that the renderer architecture is totally open in Noesis, having a custom impementation should be straightforward starting from the D3D11one. In fact, the same shaders should be compatible in D3D9.