Since he arrived in Brooklyn as a refugee from Ukraine seven years ago, Boris Khrapunskiy, a 97-year-old widower, has subsisted on federal and state disability payments, as elderly or disabled poor people in New York State have done for the past 60 years. But in June, to his shock, he received a letter from the government saying his benefits would be cut off until he became a United States citizen. The action was the result of a 1996 decision by Congress to eliminate Supplemental Security Income, or S.S.I., for most immigrants who entered the country after Aug. 22 of that...