A 2.1 mile stretch of Dorchester Avenue was “open to people” but not cars on Saturday morning as thousands turned out for the last in a series of Open Street Boston events organized by the city of Boston this year. The pedestrian zone— between Freeport Street and Gallivan Boulevard— is an expansion of a similar effort that shuts down Back Bay’s Newbury Street, first piloted in 2016. Earlier events were held this summer in Jamaica Plain and on Blue Hill Avenue.