I know a shotgun is kinda different than a cannon...
A cannon is like a cylinder bored shotgun, which is not like most shotguns out in the world. The problem with steel shot in a shotgun is the choke which gets gouged by steel shot if the barrel steel is not hard enough and bulged if the walls aren't thick enough. Since there is no choke in a cannon barrel and the walls are plenty thick, there shouldn't be a problem with using steel shot. Keep in mind that we are firing a single bore size shot with a cannon versus a handful of small shot with a shotgun. The surface of the single shot is almost an exact match for the surface of the bore so it can't do any gouging. And anyway, the escaping powder gas should hold the shot away from the bore surface most of the time.
The issue of steel shot ricocheting is certainly real, but lead shot will do the same thing. I have seen it in cowboy shooting. You still need to select an appropriate backstop whether firing a BB gun or a bowling ball mortar or something in between.