Oracle drop database - Can images stored in RMAN catalog be retained?
It sounds like you need to set up a recovery catalog in a central database, which you can read up on how to set up a Recovery Catalog and maintain it from the Oracle Database 12c Docs.  

Once you have the recovery catalog, then dropped database backup records are still maintained with their respective retention policies:

Connect to the recovery catalog:

sqlplus rman/ @catdb


Get the dbid for the database in question:

select dbid, name from rc_database order by name;


Point the recovery catalog to the database using the DBID from the previous query:

call dbms_rcvman.setdatabase(null,null,null,&lt;DBID&gt;,null);


List the backups for that database:

select * from rc_backup_files;