Some Drs remove the scalenes, some don't, it's a matter of their philosophy.
Both the middle and anterior scalene attach to the first rib, so when they remove the first rib the scalenes have to be detached at that end. In my case the middle scalene was reattached, the end stitched down to soft tissue so that it would not attach to the brachial plexus in the future. The end of the anterior scalene was left free, I believe it is thought more likely to attach itself somewhere other than the brachial plexus.
Dr Annest prefers to leave the scalenes because they provide a smooth surface for the nerves to glide over. Plus, the more you cut, the more scar matter is formed.
It most likely depends on where the compression is located, and the cause. But with the anterior scalene left free, and the middle reattached higher up, the tension on them has been released, at least theoretically, unless scar matter entraps one or both or the anterior reattaches in the worst possible place. Those are variables the surgeon can't control.
They talk about the scalene triangle, but it is really more like an L lying on it's back. Think of the short end as coming down from the neck, and the long end as the area between the collarbone (top) and the first rib (bottom). Taking the rib out will free up space below the collar bone for the nerves, but it doesn't do much at all for problems in the neck area, other than releasing tension on the scalenes as I've mentioned. That may not always be enough if there has been significant neck trauma, especially if there is compression of a vein of artery involved. Do you know if that's the case?
Has your surgeon done quite a few of these surgeries? This is definitely a time when you want someone who does these routinely, not once or twice a year!