"Do not use softened water unless it is an absolute emergency."

What is the rationale for that statement?

You don't want the salt.

Hose bibs usually bypass the softener. In my house, the kitchen faucet and the basement faucet also bypass the softener.

But how much sodium is really in the softened water? The brine solution is used to regenerate the system, the outside water isn't flowing through the brine. I know the resin beads have sodium ions which are released during the softening process, but is that really enough to jack up the sodium content in the water to high enough a level to affect plant growth? (or human health) Is the real issue the sodium, or that the other minerals are missing (removed during the softening process)? Which would also be the case with distilled water. I'm not trying to be argumentative, just reason it out.

Go ahead and water delicate seedlings with salt-softened water. Watch what happens. Report back here.

yes, please, and use cress (?) (Lepidium) seedlings, super fast and easy

I've been using softened water since moving into this house, a handful of years now. The seedlings are fine. Root growth not as robust as I'd like, but IDK if that's due to the difference in water (municipal water old house vs softened water new house) or other factors like difference in fertilizer, ambient temperature, etc.