Could possibly transform all notification messages into one bulletin board and sequentially mark them as closed?
Currently it’s not supported to display different kind of notifications in one board.
For the notifications of bell icon, once you click the icon to open the list of notifications, the number of notifications on the icon will disappear (which means these notifications are read.)
If you are referring to the task notification, it’s not supported to marked all of them as complete in once. (Some kind of task will be identified as complete automatically according to your action, e.g. once you proceed approve/reject to the pending approval record.)