What is the recommended way to do customization in reports?

All B2C Service instances provisioned on 20D or later do not have access to custom scripts for reports. These newer implementations need to utilize the rich analytics library in Agent Browser UI Extensibility Framework to customize reports. Using extensions for custom report processing provides scalability, maintainability, and high performance resulting in an improved agent experience. You can find more details on the various functions and events available in documentation for Analytics Extension.

Oracle recommends replacing custom scripts in earlier implementations with Analytics Extensions. While that is recommended, custom scripts for reports are not deprecated and will continue to work.