From what I can tell of The Witcher's system though, is that a unit only ever has one shield at a time, adding another just causes it to remove the previous one.
The difficulty is in designing something that can offer visual feedback of that shields on the unit. Since there are too many different ways one can go about designing and using shields, I've mostly left that as an exercise to the user - as one might wish to use special effects to visualize shield bars, not have a shield bar at all, or some other option that suits their map best.
The real troublesome thing is that when people can go wild with stacking shields and such, it becomes difficult to offer a good solution that would adequately cover enough unusual use cases.
For example, the shield system in the video above at 13:22 or so is only really applicable if shields always block all forms of damage, and that shields are always evaluated first applied, first damaged order. I think it looks nice though, but that could be my personal bias from having made it in the first place.
Another potential option is to select a color for each type of blocking condition, and compress the visuals into a single bar above the unit, but even then it can be difficult to read, and I'm sure someone could think of a case where it wouldn't look right. This tends to be the common approach, as seen in games like League of Legends, and Heroes of the Storm - but they also only have at most, two different types of block conditions or so, blocking all damage, or blocking magic/spell damage.
Hence, my reluctance to actually pursue a "standard" or "universal" type of shield display. If someone however has a specific display in mind for their maps and use cases, and needs help getting it implemented, I would be glad to help write something up though.
But for now, the sideways-bar-graph look easily conveys a lot of information and is probably the most useful for debugging purposes too, even if it tends to blot out everything else around it, so it'll probably remain the default unless someone argues a convincing enough case to change it.