I wrote to CDPR directly, and they answered that they plan a Linux release, and that the public announcement is going to be "soon". That was in March. Nothing happened since then announcements wise. Pretty much all public communication by CDPR ignored their Linux plans. I have no idea if that still stands or may be their plans changed since then.
Well I have a similar response (more recent than March), nothing other than to watch social media; Im really torn as to wether I should continue my playthrough on Windows (Im only just past White Orchard), or too hold out a while longer and see what comes of it.
It appears that Umbra 3D has no Linux version, and it's a major middleware component of the Witcher 3 (it's responsible for occlusion culling). So far there is no indication they even plan a Linux release, so what it means for TW3 port is replacing Umbra with something else, or waiting for Umbra to make a Linux release (which can take who knows how long).
I have seen reference to Umbra 3 working on Linux, but it maybe refering to the 'cloud' version, how that affects things im not sure, but would hate it if that meant a Linux version was always online!