[short version: Provide Gnome reward drop rates]
I may be slightly off with my interpretation here, but I would define a Loot Box as a transitional item, directly or indirectly purchasable through money, that randomly earns one or multiple items of varying value.
Chests of all kinds are recognised by the game as Loot Boxes and do display drop rates for the general rarities.
The same goes for Underworld Portals.
Dragon Eggs are not loot boxes, as all of their drops are of identical rarity, chance and value.
Orbs of Chaos are clearly a form of loot box. While the drop rate is not listed ingame, it can at least be found in the help/support website.
So far, so good.
A bit less obvious, but undeniably in the same category are Treasure Gnomes and Cedric Sparklesack. While the intermediate fight obscures it and even allows to completely lose the loot drop, these troops are “boxes”, that can earn rewards of varying value. And with vault keys being a purchasable item (directly through flash offers, indirectly through premium campaign rewards and the like), the monetary aspect seems checked as well.
However, the help pages only do list, which kind of items can be dropped by gnomes (btw., the website does not list vault keys as a possible drop on cursed gnomes; if the recent change is officially defined as not a bug for now, you might want to change that), but not the chances to receive them. I strongly suspect, that the chance on lower value drops (minor traitstones, souls) are higher than on other things, and that tarot troops have a lower drop rate than gnome troops. And the article even states itself, that the chances for keys are low.
If it is important for you to state Loot Box drop rates according to national laws, you should publish the ones for gnomes.