export can be done for objects as of time using the flashback_time or flashback_scn
export parameters. Data Definition Language statements that alter the structure of a table, such as drop/modify column, move table, drop partition, truncate table/partition, and so on, invalidate the old undo data for the table.
It is not possible to retrieve a snapshot of data from a point earlier than the time such DDLs were executed. To export a dropped table you will need first to execute flashback drop on it.
