Yes, that seems to be the distinguishing factor as to whether an economics major is a “STEM” major. The CIP number of “econometric and quantitative economics” is 46.0603. Other economics majors with CIP codes 46.06xx are not considered “STEM” majors for the purpose in question. An economics major program at a college or university may not necessarily be named by its CIP code.
If you go to College Navigator - National Center for Education Statistics , look up a college or university, and then look in the programs / majors under social sciences, you can see if “econometrics and quantitative economics” is present. Note that some schools offer both that major and “economics, general”, which means that if you want to do a DHS-classified “STEM” major, you need to be careful to choose the one that is “economics and quantitative economics” if you are doing an economics major.
Of course, this may not be applicable if your desired major is not one that straddles the border of “STEM” and “non-STEM” in DHS classifications. Note that the DHS “STEM” list includes 03.0103 “environmental studies” and 03.0104 “environmental science”, but does not appear to include any public policy type majors. You can do a similar check as above for these majors at each school on College Navigator - National Center for Education Statistics .