There is nothing that would prevent this pack from being used year round. An unusual feature that it has going for it is that the belt and yoke come in different sizes, the larger size being necessary for wearing over winter clothing.
It would be nice (and the mfr says nothing about this in their ad) to be able to buy a "normal" sized belt and yoke for 3-season use, and a larger belt and yoke for winter use.
Other than that, it's a large pack with enough room for winter clothes, fuel, hi-calorie food, and so forth.
There is a school of thought that framed packs are a bit superior to internally-framed packs for winter use, because the framed pack compresses less of the insulation on the user's back, thus reducing the possibility of said insulation becoming sweaty.