The item API was updated to show item icon IDs that can be used in conjunction with the rendering service. (source)
Given that the icon data appeared in the item_details API, I expected that requesting the icon for a random item in the database would work, but so far, all tests I did failed.
For example the “Massive Achievement Chest” with item ID `43763`. The item API gives this:
"icon_file_id":"603245",
"icon_file_signature":"6BA91E0A08689B40B09ECC3FB3C4589493E6C167",
Putting those bits together, the URL should be this: https://render.guildwars2.com/file/6BA91E0A08689B40B09ECC3FB3C4589493E6C167/603245.png – But the render API just returns a 404.
As mentioned, this happened to all items I have tested so far, both new and old items.