From time to time, you may need to delete old campaigns that are no longer in use. However, these campaigns still hold engagement data.
If you delete a campaign, the engagement data will be kept, however, the email/campaign name attached to that engagement entry will be overwritten with _DELETED in any reports.
For information on deleting campaigns, see the below article:
https://support.vutu.re/hc/en-us/articles/9442410702749-Delete-a-Campaign-the-Requirements