That way it will get a highlight effect when the cursor passes over it in the system tray. See Bug 400295 for more details.
It's not quite as described. There is an IconItem, but the parent MouseArea is intercepting the hover event and not propogating downwards.
It also uses SVGItem for some graphics which can't glow
I proposed an idea on how to glow SvgItem in Bug 416310.
Git commit 39f05d2d5b74057ed6d9a0a8a1022865ff5046e8 by Nate Graham. Committed on 24/08/2023 at 20:13. Pushed by ngraham into branch 'master'. applets/{batterymonitor,notifications}: Show CompactRep hover highlights Battery Monitor and Notifications have custom CompactRepresentations, but were not conditionally setting the active: property on hover for their custom icon items, causing them to lack the hover highlight effect that all other System Tray icons have. Related: bug 416310 FIXED-IN: 6.0 M +3 -1 applets/batterymonitor/package/contents/ui/CompactRepresentation.qml M +4 -0 applets/notifications/package/contents/ui/CompactRepresentation.qml M +5 -0 components/workspace/BatteryIcon.qml https://invent.kde.org/plasma/plasma-workspace/-/commit/39f05d2d5b74057ed6d9a0a8a1022865ff5046e8