Hi Harmish,
You could have a look at history management methods in BRepBuilderAPI_MakeShape: Generated(), Modified(), IsDeleted().
I don't know details on how split (or unchanged) edges are managed in boolean operations (note that BRepAlgoAPI_* inherit BRepAlgoAPI_BooleanOperation which redefines some of those methods and add some more). And they provide one way (from source to result) mapping, though you need the opposite. But you could do it yourself reconstructing this reverse maps.
Hope this helps. If you find some other way, please share.
Good luck.
Commenter-1
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