Following an avalanche of criticism over what some bands said was a “deportation clause” in its performance contract, the South by Southwest festival in Austin, Tex., which starts Friday, released a statement on Tuesday saying that it would amend its agreements starting in 2018. The organizers of the festival, however, furiously denied that the language in its contracts over the past five years had been designed to encourage the deportation of foreign artists visiting the United States to perform at its event.