The NBT ExtraType can be used to set a player head's owner without specifying a profile. It was the only way to specify a skull owner before player UUIDs were added (1.7 and below).

However, when loaded into Minecraft it is immediately replaced by the new SkullOwner tag. This suggests that this is an old upgrade path that doesn't use DataFixerUpper.

This causes issues like MC-164298, where ExtraType is technically still valid, but cannot be saved to NBT, and causes confusion because it only exists on the block entity.

Code analysis

Mojang mappings, 1.16.5 fabric.

public void load(BlockState blockState, CompoundTag compoundTag) {
    super.load(blockState, compoundTag);
    if (compoundTag.contains("SkullOwner", 10)) {
        this.setOwner(NbtUtils.readGameProfile(compoundTag.getCompound("SkullOwner")));
    } else if (compoundTag.contains("ExtraType", 8)) {
        String string = compoundTag.getString("ExtraType");
        if (!StringUtil.isNullOrEmpty(string)) {
            this.setOwner(new GameProfile((UUID)null, string));
        }
    }
}

This means that when an NBT ExtraType appears, it's immediately converted into a GameProfile.

This is the only location where the string ExtraType appears. In the save() method, it only puts the NBT SkullOwner.

This suggests that this is supposed to be an upgrade path, which is in the middle of the skull code instead of in net.minecraft.util.datafix.fixes.