This content applies to all versions previous to the 3.12.0
Since version 3.12.0 the Hero announcement was introduced to replace the global announcement.
Administrators can display a global message that appears in all pages; this is commonly used for maintenance purposes or platform-wide communications.
In order to do so, please go to Settings > User interface end check the Enable global announcement field. Then type the global message in the Global announcement text field and save the settings.
You can uncheck the Enable global announcement to hide the message.