I just had an interesting conversation with a colleague, who was having some trouble with her discourse notifications. She’s a hybrid user - most of the time she just uses email to participate in discussions but sometimes she logs in to start topics and like posts. This means that she often has dozens or even hundreds of notifications which she’d just like to dismiss. She did not immediately find the “Dismiss all” link on her own and when I told her about it she said it was not obvious at all.
Facebook has a “Mark all as read” link right at the top, which is probably what she is expecting. I’m sure many users have that expectation.
The top of the notifications menu is pretty full already - maybe a “Dismiss all” link could be added to the bottom right corner, next to “view older notifications”?