I'm right there with you, the new monarchy diamonds shader was my favorite and iys the one faction rally shader I did not have pop up in my inventory, I am still salty about it 2 years later.
Hey there! There is no way to obtain it as they cannot be obtained from Executor Hideo. The only way you would have had the shader in your collections was if you had it in your inventory at the time collections were introduced. Having shaders on armor/weapons did not count.