A column marked as unused may still have data, but the column name will not be displayed in queries or data dictionary views. The name is removed from the data dictionary so that a new column can reuse that name. All constriants, indexes and statistics defined on the column are also removed.
Yes if the UNUSED column hasn't affected any processes in your application and database, I'd say it'd be safe to drop. When DROP the unused column, just remember no DML can be performed on the table whilst this is occuring.. and if its a large table, that can take a while.