Where in the OFAC regulations or any IEEPA type executive order is a requirement that a U.S. bank obtain proof of U.S. citizenship from an accountholder?
That's not covered. The OFAC just requires banks to have compliance programs and says that the degree and type of compliance will vary between banks, and to "ask your regulator" about it. As the other comment said, there are less urgent ways to check for US citizenship.