First, a quick sainty check to see if I have missed something and there is indeed an analog for CheckableCommands from the Ribbon Library in the Bars Library.
Second, I would just like to voice that really liked this functionality. I'm guessing the intent of the removal is to push this functionality into a ViewModel but I don't use MVVM so not having this functionality is rather cumberson. Logically it makes sense that controls can use more feedback than just the boolean CanExecute and I prefer to lean on the visual tree rather than use explicit bindings. I know you had to smuggle it in as a command parameter which is a bit of a hack and ideally it would have been built into WPF. I'll try subclassing BarToggleButton to see if I can add this functionality back in but am not overly optimistic.