> XWiki 8.4+ It's disabled by default and needs to be enabled to activate the feature. This was chosen in order to not clutter the UI by default, considering that not all users would need this feature. In the future, it should be moved as an optional extension to install.

from https://extensions.xwiki.org/xwiki/bin/view/Extension/Message%20Stream%20Application/#HConfiguration

See https://jira.xwiki.org/browse/XWIKI-10543

This issue is henceforth low priority.