Where does the function of Identity Management should sit within IT organization?

Identity is a core foundation for all of cyber security; this belongs where the focus is security and not just making it work which is the main focus of IT

Identity and access management functions belong within an identity and access management team. This team aligns under the CISO.

Identity Access Management should belong under the CISO in order to meet NIST requirements.  Also Security Operations, GRC, and IAM work together.