This menu item is shown if the app is detected to have an AppStream ID, which we find by looking for AppStream components that match its .desktop file name in actionlist.cpp (https://invent.kde.org/plasma/plasma-workspace/-/blob/master/applets/kicker/plugin/actionlist.cpp#L433).
This produces false negatives because the .desktop fie name and the AppStream ID don't have to match and this is a perfectly valid state of affairs. When this happens, the app's context menu in Kicker, Kickoff, and Dashboard is missing the "Manage or uninstall addons" menu item.
This affects for example System Monitor, whose desktop file name is org.kde.plasma-systemmonitor.desktop, but its AppStream ID is org.kde.plasma.systemmonitor.
We should find a more robust way of getting an app's AppStream ID, since assuming it matches the name of the .desktop file is not reliable.
Fixing this would also generically fix Bug 458812.
This was traced to an upstream bug: https://github.com/ximion/appstream/issues/431.
Narrator: It was not an upstream bug
In fact the originally reported bug itself was faulty.