I've been wondering for awhile now about which version of Thief is cannon: Thief: The Dark Project or Thief GOLD. I know Thief 2 and 3 make references to characters from Thief GOLD, but it never explicitly states if Garrett visited them, so it could just be flavor for people who played Thief GOLD or is it Looking Glass and Ion Storm's way of making them both cannon?

While I do not remember the finer details in the flavor texts and conversations, I would say that both the vanilla and gold version could very well be canon since very few of the events that are exclusive to Thief Gold are referenced in Thief II. To the vanilla players, it's just extra flavors, but a nod to a previous heist to the gold version players. I guess they did not want to alienate those who did not play the gold version.

I read that the canceled Thief II Gold would have made more connections with the events exclusive to Thief Gold, one planned mission set in the slums would have been a follow up to the Ramirez/Thieves' Guild missions, and another had Garrett paying a visit to the Hand Brotherhood's necromantic rivals in their mountain castle. That could have led to a second, gold only continuity that could very easily still be compatible with Thief III, had the original plans for the series not been so altered with the closure of Looking Glass Studio.