It's likely impossible to delete data from many websites / webapps.

In a previous job we had the following
* Deleting a record set a deleted flag in the db to true
* Tertiary record of who changed what is recorded
* Application logs recorded requests which would include the data in question
* HIPAA audit trail service
* DB snapshots were taken daily / weekly, etc
* Separate data warehouse for BI reporting

It would have been a monumental task to actually delete someone's data.