This WeakAura will track a specific shield. I didn't change any of the display settings on it so it's meant to be styled and moved where you want it.
This will track a specific shield and not any other shields. By default that is Ice Barrier. But you can change the shield you are tracking at the top of the config in the Actions tab for different classes like Priest's Power Word: Shield.
If you would rather a bar use my other WeakAura: https://wago.io/Nyxwnh1kM
If you would rather use text use my other WeakAura: https://wago.io/NkWlcpk1M
I made a healthbar with shield bar (UnitGetTotalAbsorbs) for my own UI but someone in my guild wanted a bar to track Ice Barrier specifically. The game doesn't have a function to find the absorbs we have left on a specific buff so this is a little more complicated than my other shield bar.