I think it would be great if ownCloud users would be informed about new major features or changes after an update was applied. This way users can be assisted in discovering new features and don't have to discover them 'by accident' or by reading the changelog.
What's your opinion on this?
Sounds good. To avoid duplicated work maybe the release notes from https://doc.owncloud.org/server/9.2/admin_manual/release_notes.html could be shown in that welcome screen.
The other way around could be also possible. Include the release notes in ownCloud itself and auto-generate the release notes https://doc.owncloud.org/server/9.2/admin_manual/release_notes.html from it like its done with the config.sample.php.
Nice idea. One more thing to add: In my opinion we should provide a changelog there which can be understood by non-technical users as well, so maybe just TOP5 highlights or something similar.
As a workaround: You can use the announcement-app to inform users yourself. A general notification can lead to confusion when you haven't enabled some of the new features.