Maybe that’s skipped because it’s part of the basic Foreman release. Perhaps the Katello release notes should state that the Foreman release notes also apply.
For what it’s worth, the next release will introduce a PostgreSQL extension and by default we only build it for the Red Hat SCL. If you use external packages, you need to make sure the extension is available. We’ve also seen incompatibilities in with Pulp 3 where postgresql-python3 was built different. See Katello 3.15 upgrade failure with "python3-django-admin migrate --noinput" as well.