It's pretty obvious that the answer is no. Why? Because the 9 speed cassette is the same width and an 8 speed one. The spacing between the cogs is thus different, i.e. tighter on the 9. It takes a narrower chain, too. So, shifting on an 8 speed lever up front, what's the effect going to be? It'll shift the derailleur the 8 speed distance. And? You'll get away with it on most cogs until the tolerance is exceeded. This has happened to me, .... after taking 'advice' on these blogs. :))

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? :))