AAMC had good reasons to use education as a proxy for socioeconomic status. While there are outliers, on a population basis when looking at thousands of medical students, for the most part, family income is correlated with parental income. And while many parents with MD, JD, PhD, MBA, etc are making big bucks, we also know that there are circumstances where due to injury, illness or economic downturns a family may not be doing as well as expected given parents' education. (Heaven knows, many college instructors are cobbling together part-time jobs at a string of colleges as tenured positions become more scarce.)
There is no longer a "disadvantaged" check box on the AMCAS application. Every applicant is given the opportunity to provide information about childhood and family.
This information is used to paint a more detailed picture of an applicant and their life experiences. It isn't a substitute for a financial aid application.