WASHINGTON—President Donald Trump’s order halting travel to the U.S. by people from six countries remains on hold by the courts, but the number of admissions from these nations has dramatically fallen nonetheless.

Compared with a year earlier, the number of people admitted from Iran, Libya, Somalia, Sudan, Syria and Yemen was down by about half year-over-year, according to data released Wednesday by the Department of Homeland Security for the months of March and April.

Data...