It seems that we should set leader 05 to 'd' when filling deletedbiblio_metadata.metadata. See also bug 30888. The routine $authority->move_to_deleted sets the leader position 05 to 'd' for authorities.
I think that would make sense. When using OAI-PMH these records I believe are harvested with the 'd' already.
We probably would want a simple restore routine if we add this.. that undoes the 05 change if restoring a biblio from deleted. I know people do restore using SQL and this change would make that harder.