Currently, a notification is considered "unread" when it was created or updated after the last time the user opened the history.
To alleviate this, when the notification popup is hovered, e.g. because the user wanted to drag an image out, select some text, or open the "More" menu, it is considered read. It will still end up in the history but will not needlessly show the "unread notification" bell icon.
Moreover, job finished notifications are always considered read, as they are either from long-standing jobs which the user is likely aware of, or confirmation of short tasks the user doesn't need explicit confirmation about. For example, extracting an archive in Dolphin, I get an "Extracting (Finished)" but I don't want to explicitly acknowledge that notification because I can just enter the folder that was just created in Dolphin.
This surely doesn't fix the underlying problem of notification expiration and history, and the user might just interact wiht some tray icons or the clock and not actually deal with the notification popup but this is imho an acceptable stopgap until a proper solution is found, which won't happen in time for Plasma 5.17 anyway.