Does SAP Integration Suite, managed gateway for spend management and SAP Business Network Mapping Tool Support S/4HANA Cloud Custom Mapping ?

Yes, It is now supported. To enable this in your Managed Gateway for Spend&Network Mapping Tool you need to log Ariba Case with WSDL file of the Document Type you need custom mapping. In the Case as a next step a new Change Request (CR) will be created internally to enable the custom mapping by the Engineering Team.

SAP Integration Suite Managed Gateway > Managed Gateway for Business Network