Metropolitan Areas Protection and Standardization Act of 2021 or the MAPS Act of 2021 This act limits the automatic application of, and directs the Office of Management and Budget (OMB) to provide information about, changes to the standards for designating a core-based statistical area (CBSA). The standards are used to delineate metropolitan and micropolitan areas for statistical purposes. Specifically, any change to the standards of CBSA delineations (1) shall not apply automatically for any nonstatistical use by any domestic assistance program, and (2) shall apply for such uses only if a relevant agency determines that the change supports the purposes of the program and is in the public interest and the change is adopted through rulemaking procedures. The OMB must ensure that any change to the standards of CBSA delineations are accompanied by a public report that explains the scientific basis, criteria, and methodology for such change and the opinions of experts in statistics and demographics who were consulted regarding the change; are not influenced by any nonstatistical considerations; and are not applied automatically for any nonstatistical use by any domestic assistance program. Additionally, the OMB must collect information on the uses of CBSA delineations by domestic assistance programs for purposes including prime recipient and subrecipient eligibility for, and distribution of, any federal service, benefit, or funding. The Government Accountability Office must submit a report assessing the information collected by the OMB pursuant to this act.