System tray, system tray flyouts, taskbar and application launcher have their own settings window, but this is way too decentralized. There could be new categories in the Personalization section of System Settings, for clipboard, panel/taskbar, system tray, etc.
This is something that comes up periodically. When I was first using Plasma, I asked for the same thing, in fact! But there is a perhaps non-obvious issue that would make this very challenging to implement in a satisfying way: Plasma lets you have multiple "desktop containments". Each activity and screen gets one. Each desktop containment can have its own set of widgets and panels. And each panel can have its own set of widgets too. For the simple case, you have one screen and one activity and one panel and the default set of applets, and the settings for these could be put into a System Settings page. But the moment you deviate from that, the UI to manage them becomes more and more unmanageable and annoying, tnd it becomes harder to remember which applet you're configuring, because of the indirection involved. It's not really something we can do given the current flexible approach to desktop layout, sorry.
Understood.