When a system of ornamental or decorative street lighting has been continuously maintained in a residential neighborhood in a city of the metropolitan class for forty years or longer, it shall be the duty of such city to preserve and maintain such lighting system unless the city council votes by a four-fifths majority of its members to discontinue such lighting. No special assessment of any kind shall be made to the property owners within an ornamental or decorative street lighting system for the costs of any preservation or maintenance of such system.