On Friday June 3, as the Jewish holiday weekend of Shavuot began, an unusual website appeared: It showed a map of Massachusetts chock-full of dots interconnected by multicolored lines, like the corkboard of an overzealous detective or, perhaps, a conspiracy theorist. Each dot represented an institution — a school, a corporation, a police department, a nonprofit — and the lines showed their ties to one another and to Jewish organizations or philanthropists. The institutions and their purported relationships were highlighted, the Mapping Project’s anonymous creators wrote, “so we can dismantle them.” In the Boston-area Jewish community, the Mapping Project has...