Adding anti-shield items isn't a good answer

The idea reminds me of grievous wounds. I get that it exists as a "safety valve" just in case healing gets too strong, but it never actually did its job effectively in the past. In early days, healers were extremely dominant and it was a huge problem. Grievous Wounds was introduced for this reason but it didn't change things for various reasons. Ultimately, healers had to be nerfed, followed by being reworked, and items had to be removed or reworked too. The solution ended up being nerf the specific overpowered/toxic things. Adding a generic counter to deal with what was actually several specific problems didn't fix those problems.

So it's the same thing with shields except that shields have proven to be a healthy mechanic in most cases unlike heals. Shields have a lot of clarity and are inherently a fair mechanic because they don't double as both sustain and durability. Heals on the other hand give you both sustain and durability. This is another reason not to introduce a counter. It would reduce the awesome clarity of the mechanic when there is no good reason to.

Right now there is a very specific issue. Tanks with Stoneplate + Locket. No other shields are a problem. Therefore, there is no reason to make a generalized change. The specific problem should be addressed.

If you look at history (heals and GW), a generic counter would not impact this current issue and you will end up being forced to address the issue directly.

I think that it's not purely because of the shield being huge but because the build offers so much defense and utility at the same time. Removing some of the utility would be wise considering that it is the part that seems to cause a lot of frustration. Why not make Stoneplate no longer synergize with Locket? All you really have to do is make Stoneplate give a personal shield equal to the amount of current health you gain now. So it would be a shield equal to 40%/100% of current health at the moment of activation. It doesn't need to have synergy that lets you double dip in all your health scaling stuff, it really doesn't. That's the issue. The build has hypersynergy. It's too strong. There is no issue with shields in general. It's a great mechanic.