Board Thread:Support Requests - Getting Technical/@comment-28707994-20200331064205/@comment-9605025-20200331190718

Even if you mark them as read, they will still be listed. If you want to ignore announcements from certain wikis, you can use AnnouncementsIgnore to auto-mark them as read.

As for the disappearing "mark all as read" button, are you still having that issue? If so, is it on all wikis are just some? Have you tried disabling personal and site CSS/JS to see if one of those might be interfering with the button?