Migrants used to gather by the hundreds in encampments in Ciudad Juárez, on the Mexican side of the Rio Grande, waiting for a chance to cross into the United States. But as President-elect Donald J. Trump prepares to take office on Monday, few people could be found this past week on the once-teeming embankments.

All that remained were extinguished campfires, discarded shoes, shirts and toothbrushes.

One Mexican city after another has reported a similar situation along the border with the United States, where the number of migrants has steadily dropped in recent months. The decline has been attributed largely to hardened restrictions introduced by the Biden administration and by Mexican and Panamanian officials meant to deter migration.

As President Biden came under increasing pressure during his re-election campaign to curb migration flows, he issued in June an executive order effectively blocking undocumented migrants from receiving asylum. That month, U.S. border officials recorded 83,532 illegal crossings, a significant drop from the previous month’s 117,905.

Despite the decline, illegal crossings remain higher than during much of Mr. Trump’s first term, fueling calls by the new Trump administration, and even by some Democrats in Congress, for more severe restrictions on migration to the United States.