I agree that dim shield needs a change, even if it isn't actually OP, it sure is borderline broken.
If you want any kind of proof, just take a look at the
card usage stat page. I will just point to the most used unupped cards, besides quanta generators.
That said, I'm not sure it needs to be nerved to a 2 turn duration, at least not without a cost decrease. Another option would be 3 turns with a cost increase to 8|7 or even 9|8. Think of it this way, if it were to cost 2 extra quanta would you now consider it underpowered? If you answer is no, that probably means it's close to overpowered right now.
Increasing the cost would make it harder to chain, and it makes it harder to immediately put up a new shield when your opponent does pull some PC out. (because right now, a mono darkness with 3 steals still loses against a dim shield stall, simply because 3 turns of damage getting through aren't always enough, depending on what creatures you use in your deck).