Most of the derailleurs of the 6s era had plenty of over-travel and should be able to do 8s. (in theory) However things like thicker aluminum hangers than the older steel ones push the RD outboard more, so the extra travel is wasted to the outside where you don't need it.
If you already have an old 6s RD go ahead and try. If not, but an 8s or newer RD to ensure success.
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