Tags

The tag event is a server event.

The tags event takes an extra parameter that determines which registry its adding tags to. You will generally only need item, block, and fluid tags. However, it does support adding tags to any registry, including other mods ones. For mod ones make sure to include the namespace!

// Listen to item tag event
ServerEvents.tags('item', event => {
  // Get the #forge:cobblestone tag collection and add Diamond Ore to it
  event.add('forge:cobblestone', 'minecraft:diamond_ore')
  
  // Get the #forge:cobblestone tag collection and remove Mossy Cobblestone from it
  event.remove('forge:cobblestone', 'minecraft:mossy_cobblestone')
  
  // Get #forge:ingots/copper tag and remove all entries from it
  event.removeAll('forge:ingots/copper')
  
  // Required for FTB Quests to check item NBT
  event.add('itemfilters:check_nbt', 'some_item:that_has_nbt_types')
  
  // You can create new tags the same way you add to existing, just give it a name
  event.add('forge:completely_new_tag', 'minecraft:clay_ball')
  
  // Removes all tags from this entry
  event.removeAllTagsFrom('minecraft:stick')
  
  // Add all items from the forge:stone tag to the c:stone tag, unless the id contains diorite
  const stones = event.get('forge:stone').getObjectIds()
  const blacklist = Ingredient.of(/.*diorite.*/)
  stones.forEach(stone => {
    if (!blacklist.test(stone)) {
      event.add('c:stone', stone)
    }
  })
})

Recipes use item tags, not block or fluid tags. Even if items representing those are blocks, like minecraft:cobblestone, it still uses an item tag for recipes.

// Listen to the block tag event
ServerEvents.tags('block', event => {
  // Add tall grass to the climbable tag. This does make it climable!
  event.add('minecraft:climable', 'minecraft:tall_grass')
})