It's listed with each respective ISE version. Seventh up from the bottom at this link.
https://software.cisco.com/download/home/283801620/type/283802505/release/2.3.0
ACS to Cisco Identity Services Engine Migration Application Version 2.3.0. Use this application to migrate configuration data from ACS version 4.2 or 5.5 and later to ISE 2.3.
ACS-MigrationApplication-2.3.0.zip
