The "minecraft:armor_effectiveness" effect in enchantments does not work with projectiles or indirect damage.

Using a datapack it is possible do a custom enchantment. If this enchantment has the "minecraft:armor_effectiveness" effect and its value is -1,

//unarmored.json
//...
  "effects": {
    "minecraft:armor_effectiveness": [
      {
        "effect": {
          "type": "minecraft:add",
          "value": -1
        }
      }
    ]
  }
//...

this will remove the entity's entire armor.

How to reproduce:

1. Download a pack or create one with a custom enchantment that remove the armor and enable it.
2. Summon some entities with armor (or full armor) to test.
3. Give you a trident (/give @s minecraft:trident)
4. Enchant an item with the custom enchantment (in this case: /enchant @s test:unarmored).
5. Deal damage to the entities.

You will notice that the direct damage (melee) is ignoring the armor, but the indirect damage (projectiles) is not ignoring the armor. You can use /data get entity @e[sort=nearest,type=!minecraft:player,limit=1] Health to see their health.