Removing excess HVAC ductwork for efficiency
If the duct is blocked off at the end so that no air flows through then it should have minimal effect on the system. Removing it should not be expected to make any efficiency improvement.
On the other hand, if that duct is just lost in the floor or ceiling somewhere and is discharging conditioned air into who-knows-where, then yes you might gain some efficiency by blocking/removing it so that the conditioned air goes into occupied rooms instead.
It's impossible to say how much efficiency might be improved. It depends on what fraction of the system's total air flow is blowing out through this duct and whether it escapes into the environment or stays reasonably well inside the house envelope.
All that said: it is nice to remove abandoned wiring, plumbing, duct work, etc when it's feasible to do so and there's confidence that it wouldn't be used again in the future.