That might be true in a browser context, but I think both Windows and Linux (not sure about macOS) has the concept of a "clipboard" owner or "controller", which is usually the last application that copied data to the clipboard (although Linux also have the concept of "primary" and "secondary" "clipboard"), so when someone pastes into a application you're writing yourself, you could see where the data is coming from. Of course, you wouldn't be able to see what data the data that was made into the clipboard was derived from, but you get a bit of context at least.
Edit: Actually, my memory about how it works on Linux is a bit foggy, only part I'm relatively sure about is that Windows has the concept of clipboard "owner".
