How to maintain a global index on a large table undergoing regular partition purges?
version &lt;= 11.2? No.*

But in 12c, there is a new feature called Asynchronous Global Index Maintenance.

The DROP PARTITION operation completes immediately, leaving the global index in usable state. The trick is, that index entries are not maintained, they are orphaned, and these orphaned entries will be cleaned up later. The cleanup process happens automatically in the maintenance window, or it can be started manually.

More details about this: https://richardfoote.wordpress.com/category/asynchronous-global-index-maintenance/

*: There is, but I don't think it is applicable in your case. Deleting (and NOT truncating) all rows from the partition before dropping it leaves the global index in usable state. But this is enormous extra work for the 25% of a 3 TB table.