Water softeners will take the GH to 0 but will leave the KH alone.
So my well water, which has a GH=25dgh and a KH=20dkh comes out of the softener at GH=0 and KH=20.
So if shrimp can thrive in high KH water, it'll be fine. I dont think they'll like it though.
I run the softened water through an RO unit before dumping it in my shrimp tank. That takes both GH and KH to 0. I then reconstitute the GH and KH to something considerably lower than what comes out of the well.
However, I do use softened water directly in the other, non-shrimp tank with no problems whatsoever. Again, I reconstitute only the GH in this case.