Confluence/JIRA administrators get frustrated, when they need to do the same thing differently in Confluence than in JIRA:
To add an announcement banner in confluence you can do the following things:

• Change the site layout - https://confluence.atlassian.com/display/CONFKB/How+to+add+a+site-wide+banner
• Install a plugin - https://marketplace.atlassian.com/plugins/com.wittified.atl-announcer-confluence
  In JIRA you simply set the corresponding administration option: https://confluence.atlassian.com/display/JIRA/Configuring+an+Announcement+Banner

It would be best if Confluence provides the same administration option as JIRA (and gets as well the same updates/improvements for that feature).