Sophisticated Backpacks removes backpack drop from mobs that are spawned with backpacks where these mobs are not killed by real players. For that LivingDropsEvent is used and the backpack is removed from the drops list that it exposes.

I noticed this doesn't work with mob crusher and it actually collects the backpack and after checking the code it's because the mob crusher doesn't pass in the drops dropped from the mob into the event but rather only expects additions.
The event is supposed to have all of the drops for the listeners to be able to both add and modify the drops.

Edit: Just a minor addition. In my case I would only need what crusher gets from capturedDrops after DROP_SPECIAL_ITEMS is called, but it really should be passing in all captured drops same way as it is done in LivingEntity.spawnDrops call.