When you are creating a custom enchantment through a datapack, normally the custom enchantments appear in the creative inventory. But if in the field that says "supported_item" you use an item tag that is custom from your datapack the enchanted book won't appear in the creative inventory. The enchantment will still work as intended.

How to replicate:

Create an item tag inside a datapack I placed this in: data\minecraft\tags\items\enchantable\ range.json

Create two enchantments, one that uses the supported_items field with minecraft tags and one that uses the custom tag, these are the ones I used which should work the same way as multishot: multishot_not_visible.jsonand multishot_visible.json

I placed them in:

data\minecraft\enchantment\...

Open a world with the datapack in creative and in the creative search bar write multishot. Only the visible one appears.

But if you pick a bow (because the one that uses the custom tag includes bows as supported items) and try the command "/enchant @s minecraft:multishot_not_visible" on it, it will work as intended.

The only problem is that it doesn't appear with custom supported items tags in the creative inventory.