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You can still add smelting recipes with GameRegistry#addSmelting, since smelting recipes weren't changed in 1.12. By "custom recipe", I thought you were talking about creating you own recipe class with custom logic, which isn't possible for smelting recipes. Yes, you can create your own machines with their own set of recipes.

Ahhh I think I might finally understand? Sorry if I'm being thick. So forge calls the old getDrops, which calls the new one. I need to override the new getDrops in order to populate the list - and do nothing else. I'll try that!

Look at the DefaultStateMapper#getModelResourceLocation method to see how it returns a ModelResourceLocation with the Block's registry name as the domain/path and a property string of the IBlockState's property and values as the variant. You need to extend StateMapperBase and implement StateMapperBase#getModelResourceLocation to return a ModelResourceLocation with your blockstates file's domain (your mod ID) and name as the domain/path and the property string as the variant. If your mod ID is jwa and your blockstates file is called withered_debris.json, use "jwa:withered_debris" as the first argument of the ModelResourceLocation constructor. Once you've created this class, register an instance of it for your Block by calling ModelLoader.setCustomStateMapper. Do this in ModelRegistryEvent.

You called it itemgenevirus, not genevirus

There is no registered Item with the name wllcsam:genevirus. Have you spelled the name correctly? Do you register an Item with this name anywhere?