The US release of all Buffy seasons are in fullscreen, in accordance to Joss' wishes. Those were the only versions I had and were familiar with, until curiosity finally got the better of me a couple weeks ago and I ordered the Region 2 version of seasons 4 and 5 during an online sale (which at a discounted price still cost me as much as new R1 sets would have).

Why did Joss want them released in fullscreen? The season 4 episode in which Oz comes back reveals the answer -- near the end, when Oz and Willow embrace in the van, there is an obvious shot continuity "mistake", where one of the characters -- out of frame in the fullscreen version but not in the widescreen one -- leans in to hug the other, and then the camera angle changes to a wider shot and they haven't moved yet. The series was edited with a fullscreen ratio in mind, which is why mistakes like this could occur with things that aren't supposed to be seen on screen.