Because the Venafi Trust Protection Platform stores ALL of its data in the Database, to roll back after upgrading to a major version all that is needed is a backup of the database (DB). It is also good practice to have a copy of the DPAPI key just in case you need to stand up a new server.
For example, to roll back from 15.4 to 15.3 simply uninstall 15.4 from the application server, restore the DB backup, and then install the desired version (15.3 in this case) against that database.
TPP does not include any database rollback scripts. The Database Administrators (DBA) are expected to know how to backup and restore databases.