So far I have found that the following things become visible while the player is performing one of this mod's attack animations:

• Worn skulls & heads
• Worn elytra
• Worn pumpkins
• Non-equippable items and blocks placed in the helmet slot (such as with /item replace entity @s armor.head with acacia_sapling)
• Arrows stuck in the playermodel
• Parrots on either shoulder
• Spyglasses being used, although this is only noticeable because of issue #12
• I haven't checked, but from messages on the discord it seems that this also includes capes
• The player's own hitbox, when hitboxes are enabled using F3+B

The player's shadow is also visible on the ground beneath them during an attack. I'm not sure if that's related or not, I can open a separate issue if it isn't.