Immigration policy is like a revolving door with all the policy changes. One of the most impactful was Title 42. To provide some historical context, it’s explained here and there is an attached slide show.
Essentially, Title 42 was in effect in March 2020 to address the public health crisis. An emergency health measure was invoked to expel asylum seekers and other migrants arriving at the border.
Title 42 expired on May 11th because the national COVID-19 public health crisis faded which no longer gave Title 42 the health reason to expel those seeking refugee status or asylum.
Here is a policy brief about Title 42.