It was uncovered by Lucas Guttentag, a law professor at Stanford and Yale who has spent the past four years building a database of every change that Mr. Trump made to the immigration system, no matter how small. With the help of 70 of his students, Mr. Guttentag has created what he says is an online road map to the land mines that Mr. Biden will have to find and defuse if he wants to reverse the anti-immigrant agenda that Mr. Trump campaigned on in 2016 and that was executed when he was president by Stephen Miller, his chief domestic aide.

“Reform means uprooting the tentacles of every policy and undoing every last vestige of the Trump-Miller agenda,” Mr. Guttentag said. “They can be buried under layer after layer of bureaucratic actions and then essentially devastate the system in untold ways that aren’t discovered until policies are applied in particular cases.”

That was exactly what Mr. Guttentag had in mind early in 2017 as he watched the Trump administration begin to systematically dismantle policies and procedures that he himself had helped to establish.

The founding director of the American Civil Liberties Union Immigrants’ Rights Project and its leader for more than two decades, Mr. Guttentag joined the Obama administration in 2014 as a senior counselor to the secretary of homeland security. After Mr. Trump was elected, Mr. Guttentag began to compile the president’s immigration-related actions in a database that eventually became the Immigration Policy Tracking Project.