https://www.nytimes.com/2024/04/03/opinion/sex-assigned-at-birth.html
Sex assignment goes back many decades. It isn't some recent activist terminology.
This whole piece operates on some weird logic that saying assigned at birth somehow denies male and female sex.
Before we had the appropriate tests, often doctors would rely on external genitalia at birth to assign sex. For most individuals this worked with no issue. But for a minority of people, it proved messy as they had ambiguous external genitalia or were intersex. Some girls would have very large clitorises and be assigned male, or some boys would not have apparent penises and be assigned female.
In some cases, assignment surgery would occur to make the genitalia less ambiguous and to conform with the sex assignment. There was an infamous case in the 1960s where after a botched circumcision, a male twin was given female sex assignment with a surgical vagina and vulva and had his testes removed.