That is correct, I had to remove the yanfly plugin dependencies from the demo project as I'm not allowed to redistribute them. They are still loaded in the plugin manager, so someone wanting to use this plugin can simply drag and drop the correct yanfly dependencies into the plugin folder of the demo to quickly get it up and running.
I can think of one fairly easy way to implement the flatfooted state, and that's by using the <block chance: -x> tag on a state (or <lower block chance: x> but this tag is deprecated). You'd use that in combination with passive state conditions, or other conditions that always add it to the actors at the beginning of an enemy ambush, or whatever else you want, or even add the state to targets of specific skills.
You could also use that state as a trigger to change sections of action sequences as well.