There is the
use_archived_author setting you can use in that case:
Code:
[archiveofourown.org]
## AO3 allows users to archive stories they didn't write in certain
## cases. These are indicated by showing a byline such as:
## 'Orig Author [archived by Archivist Author]'
## If use_archived_author is set true (and there's only one author
## listed), author will be set to 'Orig Author' instead of 'Archivist
## Author'. authorUrl will still point to the Archivist Author's page.
use_archived_author:true