The COMPATIBLE initialization parameter must be set to 19.0.0 or higher.
Oracle Database monitors flashback logs in the fast recovery area and automatically deletes flashback logs that are beyond the retention period. When the retention target is reduced, flashback logs that are beyond the retention period are deleted immediately.
In scenarios where a sudden workload spike causes a large number of flashback logs to be created, the workload is monitored for a few days before deleting flashback logs that are beyond the retention period. This avoids the overhead of recreating the flashback logs, if another peak workload occurs soon after.
https://docs.oracle.com/en/database/oracle/oracle-database/19/newft/new-features.html#GUID-
4DE9FA4A-BA27-434A-A68E-0B58D8FD686B