lists some of the various sources of sanctions. The regulatory parts are e.g. from the 2012 Executive Order (EO13599):
All property and interests in property of the Government
of Iran, including the Central Bank of Iran, that are in the United States,
that hereafter come within the United States, or that are or hereafter come
within the possession or control of any United States person, including
any foreign branch, are blocked and may not be transferred, paid, exported,
withdrawn, or otherwise dealt in.
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.