It is not safe to drop an index that has been monitored to not been used. What do you think of indexes created to cover the deadlock threat introduced by foreign keys when deleting from the parent table?
They can easily be reported to not been used while Oracle is using them internally to avoid locking the entire table
What do you think of indexes used by Oracle optimizer only to have a correct cardinality estimation but not used in queries? they will be reported as not used and when dropped Oracle will mess up its estimation and performance issues start kicking in.
I think this whole monitoring index using v$object_usage is not safe.