is the following flag still operational per latest CF2023 HF13? Dcoldfusion.encryption.useCFMX_COMPATAsDefault=TRUE

Yes, the flag

-Dcoldfusion.encryption.useCFMX_COMPATAsDefault=TRUE

is operational on all updates of ColdFusion 2023, including Update 13. However, the flag has been removed in ColdFusion 2025. See https://helpx.adobe.com/coldfusion/kb/coldfusion-2023-update-8.html