https://issues.redhat.com/browse/SAT-31937 reports the following issue:

Following the Satellite upgrade from version 6.15 to 6.16, the content in the PostgreSQL backup directory '/var/lib/pgsql/data-old/base/16385', which was created during the upgrade, continues to be updated regularly.

In 6.16 RNs (https://docs.redhat.com/en/documentation/red_hat_satellite/6.16/html-single/release_notes/index#new-features-installation-and-upgrade), we tell users this:

When you upgrade your Satellite Server 6.15 to version 6.16, the PostgreSQL database on the system is upgraded from version 12 to version 13. During the upgrade, a backup of the PostgreSQL data is created in the /var/lib/pgsql/data-old/ directory. You can safely remove this directory after the upgrade completes.

The recommendation to remove data-old dir is not included in the 6.16. upgrades guide. We should add it there, probably as a separate step in the upgrade procedure. Ideally along with guidance on how long to store the backup (it might be good to keep it around for redundancy for some time but probably not for too long because it will eventually become obsolete).