Interesting, as I look at the press release and see some of the celebrities listed, Madeline Kahn has appeared in the oldest clips, as far back as season 9. Maybe it'll include one of her season 9 appearances (I'd guess Sing After Me is most likely), or maybe one of the two street stories she was in, or maybe her "M-Madeline" bit from around season 19. Or it could be the celebrity "Put Down the Duckie", though if this set does have a celebrity montage or two (or however high), I doubt the press release would mention celebrities whose appearances here are only in the celebrity montage.
Most of the celebrities listed seem to be from more recent appearances, but the list of celebrities included probably isn't complete. There have been past DVDs where the press releases mentioned more celebrities than who appeared in the final product (some a lot more, I think). Of course there aren't too many unreleased earlier celebrity appearances I desire that much (I do hope it has James Earl Jones reciting the alphabet, and Lena Horne's version of "Bein' Green", and some of Judy Collins' appearances). Of course Jeff Goldblum is confirmed, and his only appearance on the show was the street story where he plays Bob's brother, so that's basically a confirmed street story.