Oh yes, most fresh corn beef still has plenty of salt in it. If you don't bother to rinse it you will get plenty of liguid in the slow cooker. Great if you add whole grain to it.
You're stating that like an assumption while I have done it and found that no, there is no salt cured meat that has enough salt in it to be enough for soup.
Perhaps you eat an extremely low sodium diet and a little bit goes a long way, but I do deliberately eat a pretty low sodium diet and that would not be even close to enough to make soup for me.
Again, there is not all that much sodium relative to volume in modern refrigeration-needed corned beef.
Liquid in a cooker does not necessarily translate into sufficient sodium. Maybe you have a unique case where the beef you're getting, IS more heavily loaded with sodium than the norm these days, but again I suggest to try it and see.
The corned beef I've bought, is not remotely close to having enough sodium to be soup without more added, unless, again, there is sweet or sour to take its place.
Let's put it in context. Google thinks that 3oz of corned beef has 827mg sodium. I think it is fair to state that any "normal" soup would have far far less than 1/3rd by volume, beef to everything else (including water).
Now let's divide 827mg by 3, and you get 276mg. It would be healthier soup, but less than half the sodium that people normally find palatable for soup. Plus I was being generous, in a typical serving of soup, you wouldn't have as much as 3oz of meat.
I'm pretty sure my numbers are in the ballpark.