If the grain is straight through the fore end, it cannot expand or contract in length (to any appreciable degree) and slotting tenons is utterly unnecessary. If the wood is curly, though, then you have LOTS of cross grain stacked up and that will definitely expand and contract quite a bit.
I have yet to hear of or see any old gun that had slotted tenons. I have, however, seen barrel pins bent all to $#@*, busted wood, and tenons pulled completely out of their dovetails.
I was making a rifle once some years ago, and had the barrel inletted and pinned with the tenons un-slotted. I had the barrel out of the stock, and set it all aside for a while. During that time, the weather changed, and we had a spell of nice, low humidity weather (rare for here), and when I went back to the gun, I found that the front two barrel pins weren't even close to lining up with their holes in the tenons. The fore end had shrunk over 1/16" in total length. Ever since then I slotted my tenons (and, of course, make fore and aft space in the stock for the tenon inlet).
I will say, it is NOT one of my favorite tasks, and I don't find it nearly as quick or "easy" as others seem to. Though technically simple, for me it is a big irritation.