I originally thought 'no'. But given that the derailleur shift distance is governed by the the shifter, (taking this as said above to be true - which it probably is), then effectively there is no such thing as a specific 8 or 9 speed derailleur. It's just 'a derailleur', i.e. a slaved arm. I'm writing this because I'm having shifting trouble, though have found all sorts reasons over the years. I have an 8 speed (older) Sora bike, but Sora seems to be  9 speed now. Why the heck didn't they call it something else? :))