When you visit this forum, you'll see at least two notification banners appearing at the top of every page. One telling you about the latest CC3+ update, the other a welcome message with a link to the posting guidelines.

Notice that as long as you are a logged in to the forums, you can dismiss these permanently by clicking the X in the top right corner of each. Don't be afraid to dismiss them, while there isn't a way for you to bring them back, the link to the posting guidelines is always found in the sidebar anyway, and if a new version of CC3+ is released, the banner will be back, you don't need to keep it around to get that reminder. It is actually better to dismiss it, as a new banner popping up is more easy to notice than small text changes in the one already there.

From time to time, there may be other banners there with important information, and some pages have their own banners. These may or may not be closable, depending on the message. If they aren't closable, then they're not designed to hang around for long though, these are typically short-term messages like notifications of upcoming maintenance and the like.

Of course, if you do not log in and just browse as a guest, these won't be closable, as that state is stored in each user's account.