If a DSpaceObject gets deleted an event will be fired. In case of a Community, Collection or Item the event contains the handle as "detail" (event.getDetail()). As the object is deleted when the event gets fired, their is currently no possibility for a consumer to get all Identifiers of the object - if are any beside its handle.
Currently I see to possibilities to handle this situation:
One way would be to enhance the event system so that a deletion event contains the metadata of a deleted DSpaceObject (or at least all its identifiers). Another way would be to enhance the identifier system so that an identifier assigned to a DSpaceObject once can be found even if the DSpaceObject is deleted. The latter one would make it necessary to adopt all existing IdentiferProviders.
For DOIs (using DataCite as registration agency) we already store every DOI we assign. If a DSpaceObject gets deleted, we mark the DOI as deleted but keep the information that it was assigned once (as a DOI should never be reassigned).