Flushing notify-osd queue?

The Wiki article you pointed to is not a currently implemented spec - it's a suggested guide for developers of features that should be implemented and how they should be implemented.

This is the design specification for Notify OSD. If you are a software developer looking for advice on making your software compatible with Notify OSD, see NotificationDevelopmentGuidelines. This specification contains some issues not yet resolved. Your feedback is welcome at NotifyOSD/Comments.

This will likely be implemented in the future, for now it's rough going when the queue piles up.