I have received a lot of questions lately about deletion of data from Office 365. Different industries have requirements to remove data, and they need to be ensured that all of their content is deleted. Sometimes the US Government will call this Data Spill remediation.
The following are the best articles you should review:
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-data-retention-deletion-and-destruction-overview
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-data-destruction
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-data-immutability
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-exchange-online-data-deletion
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-sharepoint-online-data-deletion
https://docs.microsoft.com/en-us/office365/Enterprise/office-365-skype-data-deletion