This doesn’t tell me anything.
Sure. I guess, assuming you know that somehow.
None of this tells me why people think it is a problem or game breaking.
i'm not the best at understanding all this aspect of the game but my understanding of it is the following
getting flat bonuses to AC is worth more the more AC you already have because bounded accuracy limits how high your attacks can actually roll and it's hard to find alternate ways to improve your to-hit bonuses, and armour's AC is designed around that expected to-hit range.
mages and halfcasters can, pretty easily, get a
pretty decent(17~) AC through light, mage or medium armour and maybe a physical shield, the default best AC martials can get without magical assistance is 20(fullplate+shield), the Shield spell pops an additional +5 ontop of their 17~ to bump it up to 22~, better AC than the average martial for someone meant to be squishier and who needs to take less hits, and some of the tankier casters half or otherwise, can
already get 20AC from fullplate and a shield before adding Shield ontop of it.
Shield is a 1st level reaction spell that persists until the start of your next turn, this means 1) as a 1st level spell it quite quickly becomes if not spammable, then possible to apply it liberally, 2) as a reaction it's possible to apply it off-turn without cutting into your regular action economy and 3) you don't need to spend the resource until after you already know if you need to spend it as it requires the attack to already hit as it's trigger, and finally 4) it lasts until the start of your next turn rather than merely the one attack so can potentially provide protection multiple times from just the one casting.