The underlining still appears when my mouse pointer hovers over the Boards link at the top of the page but no URL is shown in the status bar (as it does for all the other links) and nothing happens when I click. The only way to access boards I've not bookmarked is to wait for them to appear in the Hot Posts or homepage listings. First noticed this about an hour ago.
Windows XP, Service Pack 2, Firefox 188.8.131.52.
Edit: the Boards link at the bottom of the page is still working, so that's a way to access non-bookmarked boards. Of course, it doesn't involve the drop-down menu like the top-of-the-page Boards link does.