I unlocked this while destroying a red chest. More accurately, I opened the chest and accidentally destroyed it during the animation which the game may have counted as destroying the chest regardless.
I think that rather than it unlocking after a certain number of destroyed chests, it unlocks when destroying a higher value chest (which you'd otherwise basically never do).
--- I unlocked this after destroying Brother Albern's chest. I don't know how to edit the infobox to the side, so someone please do it.
--- How rare is this pickup, compared to regular Junk?
--- I've destroyed a couple red chests in rainbow mode, but I didn't unlock gold junk from that. Does rainbow mode prevent that?