A rotor assembly is provided that includes a rotor disk, a rotor blade and a root spacer. The rotor disk includes a slot. The rotor blade includes a blade root that is arranged within the slot. The root spacer is arranged within the slot between the rotor disk and the blade root. The root spacer includes a base segment, a side segment and a fracture feature. The base segment radially engages the rotor disk. The side segment is radially separated from the rotor disk by a gap. The fracture feature may radially fracture the root spacer at an intersection between the base segment and the side segment.