Also to note, the shut down of immigration in the 1900s wasn't a shut down.

It was quota was set at 2% of each nationality's population as recorded in the 1890 census. That date is revealing.

The mass migration of Southern and Eastern Europeans, Italians, Jews, Slavs, etc happened after 1890. The act anchored the quota to 1890, Congress deliberately set the baseline before those groups arrived in large numbers.

The result: nations with large post-1890 diasporas, and of course all of Asia and Africa, got tiny quotas, while Northern and Western European nations got huge ones.

I don't call this stopping immigration. Rather codified national selection. It may have helped with "assimilation". But no one can deny the maneuver was for national discrimination.