Note that Wikipedia refers to these as messages[1], not banners, which I feel helps explain the difference in how they're perceived - whereas banners are generally decorative, with contrasting design style, wikipedia message boxes use the same design language as the rest of the article's infoboxes (solid backgrounds, don't span the full content width, thin+dark border with an image or single thick colored side for emphasis)
[1] https://en.wikipedia.org/wiki/Wikipedia:Template_index/Clean...