there is such a button, I just clicked it and it works. Sadly it also disappears so cant screenshot it to show it's location.a button for "mark all as read" (I haven't seen a button like that, maybe I'm blind)
When you click WHAT'S NEW you get the following tab options slightly below: WHAT'S NEW NEW POSTS NEW PROFILE POSTS YOUR NEWS FEED LATEST ACTIVITY . If you then click NEW POSTS tab the button will appear just below that on the right.
edit: ty @JaimeAOC for doing stuff so I have unread forums
Here is a screenshot with the button visible.