I checked in the OED and it says Self-depreciatory is the older term, now being swept aside by self-deprecatory - both being correct. The fact that it's used in my text book indicates that self-depreciation is on the rise again.
So which do you use? Where do you come from? And how old (roughly) are you? And if you are not a native English speaker, which term (if any) would you have been taught?
Edit Thank you everyone. It seems clear that 'self-deprecating' is still more popular, from Australia to Massachusetts via Carlisle and India.