AFAIK the National Muzzle Loader Rifle Association [NMLRA] rules say that for competition, smoothbores can't have a rear sight, which is probably due to authenticity and to level the playing field.
However TC made their .56 Renegade smoothbore with an adjustable rear sight because it was used as hunting rifle.
Even if there's no rear sight, a shooter still uses a rear reference point to line up with the front sight to help aim better.
That might only be a tang screw or a mark on a barrel flat, or it can be difficult to shoot consistently.
The rear sight issue can cause somewhat of a dilemma about wanting to have an authentic smoothbore or not.
Some smoothbores may have a better reference point to aim with than others.