You'd expect first-party apps to handle this properly. The intention is that you probe the clipboard metadata (which doesn't display this warning) to see if the clipboard looks like it might be useful to you - and then act (or offer to act) if it does.
So first-party apps should have bought into the data-detectors stuff wholesale.
An example of this would be the url bar in Safari. If you try to paste into the address bar, you'll be offered "Paste and go" if the clipboard offers a url-like object, and "Paste and search" if it's a non-url stringy object. You'll get the "Safari has pasted from Mail" warning after you select one of these, because the contents were not exposed during the metadata operations. This is the expected behaviour.
So first-party apps should have bought into the data-detectors stuff wholesale.
An example of this would be the url bar in Safari. If you try to paste into the address bar, you'll be offered "Paste and go" if the clipboard offers a url-like object, and "Paste and search" if it's a non-url stringy object. You'll get the "Safari has pasted from Mail" warning after you select one of these, because the contents were not exposed during the metadata operations. This is the expected behaviour.