This email means that the user deleted a versioned folder in the past and now the folder is about to be purged. You can restore the folder or ignore this message and let it be erased from the system.
When the user deletes a versioned folder, it remains as deleted, until the total number of days defined in Group Policy / Retention Policy (default in 90 days) expires:
If after receiving the notification, the user wants to keep the folder, must log in as the user who owns the folder and clicks 'Show Deleted' icon on the top right, the deleted elements will be seen with the strikethrough name.
Folders can be restored via right-click / ‘Undo Delete Recursively’:
For more details about How to restore deleted file/folder? you can visit the related FAQ.
If the user wants to stop receiving these emails, must go to the Tenant Management Console / Group Policy / Retention Policy / 'Don't send email notifications when purging deleted content':