I cannot recall pictures of the late Queen Elizabeth wearing this brooch, but you might be correct.
Does anyone know when the Duchess first wore this brooch?
The Duchess usually wear brooches, at least for the more formal engagements. I suspect she finds it too much with the combination of brooch and poppies, and thus she would have worn more brooches if it wasn´t for the poppies.
On the other hand, the diamond tours (both the Wales/Cornwall tours, the Cambridge tours and the Wessex tour) have been very low-key on the glittering side, few evening events, and no grand jewelry for any evening events.