As of April 1th. 2022, AIX 7.3 is now fully supported by SAP.
https://w3.ibm.com/w3publisher/sap-on-ibm-systems/news/b3ad46f0-b4c4-11ec-9858-01c4e3151d8a
Details on supported SAP software levels:
SAP Product Availability Matrix (requires SAP login) and SAP Note 3104875 – Using SAP systems with AIX 7.3 (requires SAP login).
Further information:
https://community.sap.com/topics/aix