You only listed champions with shields. Like I said, the shield isn't an essential part of their kits. If you'd like, I can break down one by one why each of those you listed.
Although, you mentioned Mundo being a bad design. Likewise, Mordekaiser being based around his shield is also bad design.
My point was that most champions with a personal shield wouldn't be crippled from not having it in their kits, they're merely buffed above their peers by having one at all-- since for every single case, the shields are on top of other abilities, not solely shields.
But I'll name a few who definitely should have counters to their shield because it adds more than need be, and/or in prevents them from buffing the actual core abilities of the champions because they have that defensive capability.
Often bursts down her targets without needing the shield, or pokes without worry because she has a shield on a dash.
Will also often kill his opponent without much damage to the shield. It's just more defense than he needs.
Had his knockup removed, while they opted to keep the shield that all but requires you to intentionally position incorrectly to use it.
Rarely ever gets to use the damage aspect of his W on an opponent because it can't be cast if the shield pops.
While his shield isn't the main issue behind his 'unfair for 99% of players' nerfs, it is a contributing factor. While it's a cool concept to give him a shield when using his abilities in a different order, it prevents them from putting any power into his abilities because it would conflict with the overwhelming (if used in master or challenger) strength of his ultimate.
Who, as we see, received unfair nerfs to her W's armor and MR steroid despite not actually performing better than average.
Giving us an option to cut down on their shielding would both grant more room for playing better against them, and give Riot room to buff them if they're struggling, for those who play them. It's a win-win-win situation, including removing the need for enchanters to be nerfed.