Intelligence slows down your progression in the game by making you gain Intelligence points to craft certain items.

You start off with 10 Intelligence points, and you can gain points (up to 8), by crafting books, bookshelves and more. Every craft you do (excluding books and bookshelves), you may lose 1 - 3 Intelligence points.

Info:

To craft items like diamond tools or iron armour, you are required to gain some Intelligence before crafting them. Iron tools require 30 points, whilst diamond armour requires 60. This helps to slow down progression in your server since people can no longer get diamond stuff in 10 minutes, and rather requires a bit more time to get better gear.

Items like wood tools or some stone tools have been omitted from the Intelligence requirements, since adding them to it, will probably be annoying early game.

And there's more, but you'll have to play to find that out.

If you want to view the full list of items in 1.0.0 for 1.21.9, here it is:

        INTELLIGENCE_REQUIREMENTS.put(Items.NETHERITE_BLOCK, 75);

        // diamond
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_PICKAXE, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_SWORD, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_AXE, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_SHOVEL, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_HOE, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_LEGGINGS, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_CHESTPLATE, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_HELMET, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_BOOTS, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_HORSE_ARMOR, 60);
        INTELLIGENCE_REQUIREMENTS.put(Items.DIAMOND_BLOCK, 60);

        // iron
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_PICKAXE, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_SWORD, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_AXE, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_HOE, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_SHOVEL, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_HORSE_ARMOR, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_CHESTPLATE, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_LEGGINGS, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_BOOTS, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_HELMET, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_BARS, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_BLOCK, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_CHAIN, 21);
        INTELLIGENCE_REQUIREMENTS.put(Items.COPPER_CHAINS.unaffected(), 21);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_TRAPDOOR, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.IRON_DOOR, 27);
        INTELLIGENCE_REQUIREMENTS.put(Items.HEAVY_WEIGHTED_PRESSURE_PLATE, 21);

        // gold

        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_LEGGINGS, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_BOOTS, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_HELMET, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_CHESTPLATE, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_APPLE, 50);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_AXE, 25 );
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_SWORD, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_HORSE_ARMOR, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_HOE, 25 );
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_SHOVEL, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_PICKAXE, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.LIGHT_WEIGHTED_PRESSURE_PLATE, 21);
        INTELLIGENCE_REQUIREMENTS.put(Items.GOLDEN_CARROT, 45);

        // stone

        INTELLIGENCE_REQUIREMENTS.put(Items.STONE_AXE, 13);
        INTELLIGENCE_REQUIREMENTS.put(Items.STONE_HOE, 13);

        // leather

        INTELLIGENCE_REQUIREMENTS.put(Items.LEATHER_BOOTS, 7);
        INTELLIGENCE_REQUIREMENTS.put(Items.LEATHER_CHESTPLATE, 7);
        INTELLIGENCE_REQUIREMENTS.put(Items.LEATHER_HELMET, 7);
        INTELLIGENCE_REQUIREMENTS.put(Items.LEATHER_LEGGINGS, 7);
        INTELLIGENCE_REQUIREMENTS.put(Items.LEATHER_HORSE_ARMOR, 7);

        // misc

        INTELLIGENCE_REQUIREMENTS.put(Items.ENCHANTING_TABLE, 50);
        INTELLIGENCE_REQUIREMENTS.put(Items.ANVIL, 45);
        INTELLIGENCE_REQUIREMENTS.put(Items.BREWING_STAND, 45);
        INTELLIGENCE_REQUIREMENTS.put(Items.NOTE_BLOCK, 33);
        INTELLIGENCE_REQUIREMENTS.put(Items.LOOM, 20 );
        INTELLIGENCE_REQUIREMENTS.put(Items.SMITHING_TABLE, 50);
        INTELLIGENCE_REQUIREMENTS.put(Items.NETHERITE_UPGRADE_SMITHING_TEMPLATE, 70);
        INTELLIGENCE_REQUIREMENTS.put(Items.BOW, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.CROSSBOW, 40);
        INTELLIGENCE_REQUIREMENTS.put(Items.MACE, 75);
        INTELLIGENCE_REQUIREMENTS.put(Items.WIND_CHARGE, 50);
        INTELLIGENCE_REQUIREMENTS.put(Items.FIREWORK_ROCKET, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.FLINT_AND_STEEL, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.LECTERN, 30);

        // woods [

            // boats
        INTELLIGENCE_REQUIREMENTS.put(Items.OAK_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.SPRUCE_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.CHERRY_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.ACACIA_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.BAMBOO_RAFT, 25);
        INTELLIGENCE_REQUIREMENTS.put(Items.DARK_OAK_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.JUNGLE_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.BIRCH_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.MANGROVE_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.PALE_OAK_BOAT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.OAK_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.SPRUCE_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.CHERRY_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.ACACIA_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.BAMBOO_CHEST_RAFT, 30);
        INTELLIGENCE_REQUIREMENTS.put(Items.DARK_OAK_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.JUNGLE_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.BIRCH_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.MANGROVE_CHEST_BOAT, 35);
        INTELLIGENCE_REQUIREMENTS.put(Items.PALE_OAK_CHEST_BOAT, 35);

            // doors

        INTELLIGENCE_REQUIREMENTS.put(Items.OAK_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.ACACIA_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.DARK_OAK_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.BIRCH_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.CHERRY_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.BAMBOO_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.JUNGLE_DOOR, 20);
        INTELLIGENCE_REQUIREMENTS.put(Items.PALE_OAK_DOOR, 20);