The wiki says this item can still be received in two ways:
'can still be obtained by receiving a gift of the game,[2] or by activating a retail copy of the game, such as purchasing the Orange Box. '
I have received the Proof of Purchase after donating to the Rewired Rampage charity event and after the medals were sent out.
Should this be documented, was not sure if this covered in the line 'receiving a gift of the game'