Pantheon Shouldn't Block Tower Shots.
Fiora can riposte ANYTHING in the game: A Guillotine coming down on her head, a Magical Sword coming down to smite her to death, a ginormous hammer erupting from the ground - Still can't block a tower shot.
Yasuo can Windwall any skillshot/Ranged Auto in the game: Can part the freaking seas, Turn a Giant crystal arrow into a snowflake, and a high caliber rifle shot just vanishes - Still can't deflect a tower shot.
Braum can Deflect the first hit from anything in the direction of his door: Literally ANY damage - Still can't block a tower shot.
So why. Can Pantheon. Do it. Walk in with Passive up block Use stun block walk out. Un. Harmed.
I call. Buuuuuuuuuuuuuuulllshit.
Especially considering Pantheon is likely going to bully you to UNDER your tower with his Q Q Q Q Q combo... You should be able to rely on a giant tower to help you if he decides to get careless and walk under it.
shields/parry . He blocks ONE basic attack or ONE tower shot. That's his thing