The Flattening is a very simple NeoForge/Fabric Architectury mod that allows datapacks (and other mods) to easily add in-world anvil flattening recipes.

Video, because it's hard to explain:

Recipe JSON Examples

Block Flattening:

{
  "type":"the_flattening:flattening",
  "input":{
    "id":"minecraft:copper_block"
  },
  "output":{
    "id":"minecraft:copper_trapdoor",
    "count":12
  }
}

Block Flattening Schema:

type: "the_flattening:flattening" [required as shown]
input: {
  id: [Item ID String, must refer to a block, required]
}
output: {
  id: [Item ID String, required]
  count: [Integer, Optional, defaults to 1]
}

Entity Flattening [1.0.1+]:

{
  "type":"the_flattening:entity_flattening",
  "input":"minecraft:cow",
  "output":{
    "id":"minecraft:milk_bucket"
  },
  "shouldDropEntityLoot": true,
  "shouldDropXp": true
}

Entity Flattening Schema [1.0.1+]:

type: "the_flattening:entity_flattening" [required as shown]
input: [Entity Type ID String, required]
output: {
  id: [Item ID String, required]
  count: [Integer, Optional, defaults to 1]
}
shouldDropEntityLoot: [Boolean, Optional, defaults to false]
shouldDropXp: [Boolean, Optional, defaults to false]