Teleporter is the vanilla Minecraft class, while ITeleporter is the forge alternative. There is no reason the extends Teleporter, since everything you get from it is a bunch of portal spawning logic. Your VoidTeleporter should work as desired if you implement ITeleporter, but keep the placeEntity override, since as the documentation in ITeleporter says, a portal will not spawn if you supply the Function with false.
I wouldn't say I know enough Java to pr Forge, so I will instead make my suggestion here.
I think it would be useful if there was a ShieldBlockEvent of some sort, which is fired when an an attack is blocked by a shield. I would assume you could get the DamageSource of the attack along with the shield item from the event.
Maybe because this is my first Forge-Mod???
I did not knew about this feature and tutorials on YT also did not speak about it.
I will check it out, but what i were reading for now seemed to be pretty usable just wanna understand it before using it