In my Profile/Notifications the button “Dismiss All” is inactive.
Do you talk about the daily E-Mail summaries?
You can control them in Preferences -> Emails -> Activity Summary
Ah, I see. I am not sure if you can delete them at all. But why do you wish to do so?
Counter-question: Why should I collect them?
Today there are 10, in a week 50 … I like it clean and see no profit for me. I just want to be able to delete the read notifications.
It is a bit like a history log… (I agree, not too useful)
You can set the Notification page’s filter to display only unread, but this won’t make them disappear in the avatar-menu.