Historically, many wikis have been known to use the classes successbox, messagebox, errorbox and warningbox inside templates. Confusingly, these classes were added inside MediaWiki core code to markup important system messages that serve a different purpose from artivlr notices, such as the error that displays when you login with an invalid password. In most cases was unintended (T270796#6714564).
Because of this we will be removing all CSS and HTML relating to these classes in MediaWiki core around April 2022.
If you are a template developer, please check that if you using any of these classes that you are not relying on any of the styling rules that come from core to minimize any disruption to gadgets/scripts.
If you are intentionally using these classes and wish to continue to do so, a new method mw.util.messageBox is being considered to support your use case. If it is added, then this method would be considered stable just like mw.util.addPortletLink. Please let us know. Use of the classes is a possibility in the mesn time with the caveat that the markup and classes are subject to change in future.
If you have any concerns or questions please feel free to comment on this Phabricator ticket.