<www-apps/owncloud-11 should depend on dev-db/postgresql:9.6 with USE="postgres" https://github.com/owncloud/core/issues/29997 https://github.com/owncloud/core/issues/31424
Thanks for the report and sorry for the long delay answering, this one somehow was not in my list. While it is still valid (as mentioned upstream) that postgresql >= 10 is not supported, this can not be easily added as ebuild dependency, the database can be on another system. This is why DB dependencies are only there to enable relevant PHP modules. Hopefully this can finally be updated upstream (though they heavily recommend mariadb)