Currently, when a document is no longer on our repository, we keep its UID and it is the only information we have about that document.
When these deleted documents are referenced in some elements (e.g. nuxeo-document-suggestion) we can only present the UID, but it should be clear to the users that the document is no longer present and not accessible.
It may also happen with users/groups, directories.
On the other hand, we may need to rethink the API for these elements in order to provide options to hide deleted documents and, probably, to change the way they look when shown.