The %PastClipboardFlavors% token (v11.0+) returns the flavors of a specified past clipboard. It has the form %PastClipboardFlavors%<Index>% where <Index> is a calculation which specifies which past clipboard to use.
The indices start at 0 for the current system clipboard, 1 for the previous clipboard, and then onwards for as many clipboards as Keyboard Maestro has in its history (the default maximum is 200).