The list of non-armour items that can be put into the head slot is hardcoded in net.minecraft.entity.EntityLiving.getSlotForItemStack().

What does that mean? Couldn’t you make it possible to wear endermen heads?

Because a block is a block and the enderman head isn't

@PapiDimmi as Henry wrote - you can only wear helmets or a small finite set of items defined by vanilla Minecraft.

So to avoid further misunderstandings => can't be fixed.

You can literally wear any block on your head, so I don’t understand why this would be impossible, but okay.

Because a block is a block and the enderman head isn't

Yes, it is. It can be placed, can’t it?

It's not about it working in the head slot, it is about putting it there. And any inventory that has armor slots is using the method I quoted above to determine if any item can be put there.

I can disable that check in the CapBank, then you can put any block---including the enderman head---there. And they will work. However, that still won't allow you to put it there using e.g. the player inventory.