I have a recent Omeka installation and have tested a composer -dry-run (with --no-dev). I have seen that many libraries, especially from Zend Framework, are slightly outdated. Could I safely update them with Composer or is Omeka based on a specific version number?
You can update the Composer dependencies on an Omeka S install if you wish, but this won’t always work… the version specifications should guarantee that there are not compatibility issues but there are sometimes problems in practice.
The intended situation for upgrading those dependencies is that they get updated when new versions of Omeka S are released.