Section 80. A town which accepts section seventy-three or has accepted corresponding provisions of earlier laws or has accepted any special act authorizing the creation of a board of survey therein may establish, in the manner provided for the laying out of town ways, the exterior lines of any way, the plan of which is approved under section seventy-four or seventy-five or under such special act; and thereafter no structure shall be erected or maintained between the exterior lines of the way so established, except that buildings or parts of buildings existing at the time of the establishment of said lines may remain and be maintained to such extent and under such conditions as may be prescribed by the board of survey of such town. Lines established under this section may be discontinued in the manner provided for the discontinuance of a highway or a town way. This section shall not apply to cities.