Deleting segments (and tags) will not prevent you from viewing sent campaign reports. In the event that you delete a segment/tag, you are deleting the grouping of contacts, you are not deleting contact records or campaigns sent to the deleted segment or tag.

However, when you delete a segment/tag, the name of that segment/tag can no longer be referenced in reports. For example, in the Tracking Statistics report you will see a blank value in the Segment column when the associated segment/tag has been deleted.

If segment/tag identification is important to you in reports, alternatives could include 1) naming your campaigns in a way that indicates the segment or tag or 2) creating an archive folder in your segment or tag dashboard and move unused groups to that folder to remove them from view.

If you choose to use campaign names to provide context to segments/tags sent to, you can rename sent campaigns. One way to rename a sent campaign is from the Tracking Statistics report. Click on the campaign ID (far left column) for the campaign. Edit the "Mailing Name" field on the next screen and click "Save".