… Millions of undocumented workers pay taxes, improving the financial outlook for federal programs like Social Security. The I.R.S. has encouraged them to submit tax returns using a nine-digit code called an individual taxpayer identification number. Immigration activists and tax lawyers said they had long trusted that the I.R.S. would protect the confidentiality of migrants’ tax information, which includes where people live and work, and details about their families.
The change in approach from the I.R.S. could drive more immigrants to stop filing their taxes and seek under-the-table jobs, economists said.