There's no leagues where they aren't restricted to 1 or banned, except the casual playgroups of rich jerks.
Well, we printed them in my school, and put them in foils and used normal cards in the back (to have normal background), just to test how overpowered the first decks in magics history were. (And I can tell you, you wouldn't be able to imagine a Power Nine+Old Sliver deck...) (for those who don't know, slivers are little evil creatures which are buffing each other. There are old and new ones, and the old ones were much faster to play...) ---Similar to that, I would also love to test how overpowered sundials were before nerfing started. ---> well I guess that is pretty of topic.
Well, knowing what overpower can really mean, I can't actually sence that with dim shields (or aether element as a whole) Yes, Aether, and dim shields are stronger than other cards and elements in this game, and yes, dim shields do restrict the design of other decks (since 1. you need more PC for dim shields than for any other card, and you would still be able to overcome any other shield without PC or Momentum.) and yes, Dim Shields could really be nerfed and would still be viable, and yes, it would probably make the game more balanced, but it's not a gamebreaking card which needs nerfing no matter what.