Consider this example inside data\ic2\ic2_recipes\scrap_drops folder:

{
    "drop": {
        "item": "minecraft:quartz",
        "size": 1,
        "type": "ic2:item"
    },
    "chance": 90.0
}

It gets added to the game, but is completely ignored when opening Scrapboxes

Versions:
IC2Classic: 1.19.2-2.1.0.1
Forge: 1.19.2-43.3.13