A shield remains in a blocking state after a player holds up a shield and travels through the Nether Portal. However, the shield animation itself is in its default lowered state and the player is able to walk around at a normal speed and freely attack and interact with blocks and items as if their shield was down. Essentially giving the player a wall of invincibility in front of them.

This can simply be recreated by holding a shield up while entering a Nether Portal, and can be seen in action if you are to be attacked by a hostile mob. Raising the shield and lowering it, or drawing and firing a bow will remove this effect.