There's obviously a lot of variables that can affect this. A lot of it boils down to - What is an ethical shot?

Civil War snipers often made kills with .45 cal Whitworths or even Springfield muzzleloaders at 800 - 1000 yards. So a muzzleloader can certainly kill a deer at extreme ranges, but that doesn't make it ethical to shoot that far. Those snipers also missed and wounded soldiers probably far more often than they hit at those extreme ranges.

I would say that a modern muzzleloader with a scope in the hands of a good shot (another important factor) will often have an effective, ethical range of 150 - 200 yards. My ML is sighted in to be dead-on at 100 yards, and drops about 5 inches at 150. I would take a shot out to 150, but would prefer 100 yards or closer. I know of some who can routinely make hits at 200, and would never say anything bad about someone who shoots that far, but practices at that range and knows what their gun will do.

Past 200 yards, there aren't many loads that will retain enough energy and trajectory to make ethical shots, in my opinion.