I can’t find anything online. I know other surveys, like NHANES, have one.
As per this advanced letter that is sent to NHIS participants, The Ethics Review Board of NCHS reviews survey content and methods for the NHIS. IPUMS data harmonization of the NHIS is not considered human subjects research, and is therefore not subject to an ethics IRB review the same way that human subjects research is. The data available via IPUMS NHIS is derived from the public use files released by the National Center for Health Statistics, which have been deemed appropriate for public release by the agency. IPUMS NHIS requires users to adhere to terms of use that reflect the terms required by NCHS.
Thank you for your help, Kari!