Summary:
Holding items enchanted with thorns in your hand in 24w18a does not work the same as if you were wearing them.
In version 1.20.6 and below you could hold items enchanted with thorns so that entities would take damage when attacking you, related report resolved as WAI - 06/Oct/23 - MC-73710.

Steps to reproduce:

1. Switch into survival mode and hold an item enchanted with thorns by using the command provided below.

   /give @p netherite_chestplate[enchantments={levels:{"minecraft:thorns":255}}] 1

2. Summon an entity that will attack you, for example, a husk.
3. Let it damage you a couple of times.
4. Take note as to whether or not holding items enchanted with thorns in your hand works as if you were wearing them.

Observed result:
Holding items enchanted with thorns in your hand would not work as if you were wearing them.

Expected result:
Holding items enchanted with thorns in your hand works as if you were wearing them.

Video:
24w18a_thorns.mp4
1.20.6_thorns.mp4