⛔Whitelist/blacklist items

How to make backpack accept only some items?

You have to use the whitelist attribute. For example this backpack will accept only redstone related items:

redstone:
  display_name: '&fRedstone Backpack'
  item:
    type: HEAD
    texture: 'eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvYmRhOTExNDM3YjRlY2ZhYTNjMTg5NDE2MjIxN2MwMWI2OGE1NWM4OWJiMmY0ZDQ5MjczNDVjZTVjNzk0In19fQ=='
  size: 5
  whitelist:
    - REDSTONE
    - DISPENSER
    - NOTE_BLOCK
    - STICKY_PISTON
    - PISTON
    - TNT
    - LEVER
    - STONE_PRESSURE_PLATE
    - OAK_PRESSURE_PLATE
    - SPRUCE_PRESSURE_PLATE
    - BIRCH_PRESSURE_PLATE
    - JUNGLE_PRESSURE_PLATE
    - ACACIA_PRESSURE_PLATE
    - DARK_OAK_PRESSURE_PLATE
    - REDSTONE_TORCH
    - STONE_BUTTON
    - STONE_BUTTON
    - OAK_BUTTON
    - SPRUCE_BUTTON
    - BIRCH_BUTTON
    - JUNGLE_BUTTON
    - ACACIA_BUTTON
    - DARK_OAK_BUTTON
    - OAK_TRAPDOOR
    - SPRUCE_TRAPDOOR
    - BIRCH_TRAPDOOR
    - JUNGLE_TRAPDOOR
    - ACACIA_TRAPDOOR
    - DARK_OAK_TRAPDOOR
    - OAK_FENCE_GATE
    - SPRUCE_FENCE_GATE
    - BIRCH_FENCE_GATE
    - JUNGLE_FENCE_GATE
    - ACACIA_FENCE_GATE
    - DARK_OAK_FENCE_GATE
    - TRIPWIRE_HOOK
    - TRAPPED_CHEST
    - LIGHT_WEIGHTED_PRESSURE_PLATE
    - HEAVY_WEIGHTED_PRESSURE_PLATE
    - DAYLIGHT_DETECTOR
    - REDSTONE_BLOCK
    - HOPPER
    - DROPPER
    - IRON_TRAPDOOR
    - OBSERVER
    - IRON_DOOR
    - OAK_DOOR
    - REPEATER
    - COMPARATOR

How to blacklist items from a backpack

You just have to add the blacklist attribute and specify materials.

example_1:
  display_name: '&fexample'
  item:
    type: HEAD
    texture: 'eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvMmEzYjM0ODYyYjlhZmI2M2NmOGQ1Nzc5OTY2ZDNmYmE3MGFmODJiMDRlODNmM2VhZjY0NDlhZWJhIn19fQ=='
  size: 2
  blacklist:
    - BARRIER
    - BEDROCK