It seems that there's no shadows for contextual menus while using the gtk theme engine. Is this intentional, a bug or just a missing feature?

Missing feature in Gtk - the applications resp. GUI styles don't hint the need/interest to be shadowed.

i didn't know ( i don't think) that kde and qt applications were able to use the gtk toolkit.

By "gtk theme engine" you meant "kwrite --style gtk+"?
Same issue (doesn't set resp. hints) but other addressee - it's a Qt built-in style.

That aside, of course you can link Qt and gtk+ at the same time, eg. the gtk-qt style does so.

Ok then, this seems to be a kwin bug due to lack of interest . Kwin can't properly detect a menu.

As previously mentioned, this is a Gtk bug, not a KWin bug as applications need to indicate which areas are menus - the window manager has no idea what the application itself is, or where elements are positioned within it.

The problem is not to know whether a Window is a menu, but to know whether the window wants to be shadowed at all (very simple case: it paints "shadows" etc. internally)
Next problem for KWin is to know about the light concept of the current theme (mostly N-S vs. NW-SE) why we require the UI elements to indicate that
a) they want a shadow drawn by the WM
b) what this shadow looks like and where to place it

Shouldn't the composite window manager have the last word on how, where and when shadows should be rendered?. The rest of the window managers can do it properly.

Last word - sure.
General claim? No.

We removed the generic shadows since there were strong reservations in the oxygen team about their visual appeal. So we enabled the clients to hint:
- shadow me
- make it look this way

Show me how the "Other window manager" *properly* shadow the oxygen transparent theme with rounded borders....