It's still listed in the updates on the Gran Turismo 6 product page so we should definitely still see it. When that will be, I've no idea.
Eagerly waiting for it myself as it's one of the key features I bought GT6 for. I hate the content in GT6 coming in dribs and drabs as I'd prefer to have still had GT5 support and buy GT6 as a full game, including all the planned updates integrated already.
I'm getting a bit worried about the timing of it's appearance though. If it arrives in an update close to GT7s release, I doubt I'll bother with it as I'll be grabbing a PS4 and GT7. Would you be able to go back to GT6 after seeing how good GT7 looks? I doubt I could but then I'd feel a bit cheated. I wasn't expecting a year long wait for B Spec.