Are Clean and Drop Scripts Mandatory in the IFS Upgrade Process?

I have reviewed the official documentation regarding Clean and Drop scripts in the upgrade process:
https://docs.ifs.com/techdocs/25r1/070_remote_deploy/040_upgrading_system/040_deploying_upgrade/040_clean_drop_scripts/
I would like to clarify whether executing the Clean and Drop scripts is considered a mandatory step in all upgrade projects, or if it is optional and dependent on specific conditions (e.g. project scope, environment, type of customization).
In actual implementation projects, are there cases where these scripts are intentionally not executed, and if so, what criteria or risks are typically considered when making that decision?
Any guidance or best practices from the field would be appreciated.
Thank you.