When it comes to applications there is a wide variety of implementation of system tray icon behavior. That being the application hidden and only exposed through an icon in the task bar. Some application show their main application window on left click, but don't do anything else. Some application toggle their main application window on left click. Some application provide a context menu on left click Some application do nothing on left click and only provide a menu on right click. Some application do nothing on right click and provide a menu on left click. Linux is a vast community but it seems that plasma is an entity who has wide adoption and supports the system tray. It isn't reasonable to expect everybody to ever follow any guidelines. But if someone does care, wants to research expected behavior, it would be nice to have some guidelines. Maybe the plasma HIG are in a position to suggest a preferred behavior.
We do: https://develop.kde.org/hig/status_changes/#system-tray-icons Overall our guidance is "don't do it". The fact that every app implements their own uniquely bad UX for system tray icons is the reason we make that recommendation. THe UX basically cannot be made any good here, even if it were actually consistent between apps (which it never will be).