This fixes many issues I've experienced in terms of notification syncing with KDE connect. Though this still is the best computer-phone integration app I've come across.
I've hidden all media notifications, as they reappear every time the current song changes, starts or stops. Media player state is already synced by MprisPlugin.
I added two user-configurable options that are both enabled by default that can be accessed in Plugin Settings->Notification Sync Settings. One silences existing notifications during connection. This prevents your phone from connecting and 30 notifications appearing on your computer which you have to close, and can take quite a while to disappear on their own. These notifications are still sent, but with "silent" sent, so they don't appear on your screen.
Another silences redundant notification updates. I've noticed often apps "update" the notification but the text and content is exactly the same, causing an duplicate notification to pop up. This keeps track of which notifications have been sent, and silences any updates that are the same as older notifications.
I had to move "Filter Notifications" down a menu. So to access it you now go to the notification sync settings, and click "filter notifications".
Enable "Silence Existing Notifications" if disabled, then disconnect KDE connect (leave wifi, close daemon, etc) and then reconnect. No notifications should pop up but they should be visible in the KDE connect menu in the taskbar. Turn it off, and they should appear upon connecting as they did before the patch.
Play music, no notification should appear on the desktop.
Enable "Silence redundant notifications" if disabled, create two notifications with the same text and title, and check only one shows. Turn it off, the opposite should occur. Or, if you have some apps that "update" their notifications with no change, check they no longer appear.