In my short time here, I already got a lot of Discourse notifications:
According to this meta.discourse post,
Notifications go away when you interact with the cause of the notification, either by clicking the notification or navigating manually to the cause […]
However, my notifications don’t go away like that, and in addition to that, the
Dismiss All button is disabled on my page https://discourse.slicer.org/u/jondo/notifications.
Does this work for you?