You mean strictly valid in closed systems but whether or not the system allows photon dissapation of heat,
momentum is still propagated through the system through phonon transmission i.e. sound waves
Even if the energy is not conserved the momentum can be conserved. But, if there are air then the system of two colliding objects is not closed because the momentum is transfered to the air molecules (air resistance, Newtons third law). If the system is isolated and the collision is inelastic, the energy is not conserved (heat production etc.) but the momentum is conserved.