You can't take it with you, but you can't leave it for someone else either: Valve says you aren't allowed to bequeath a Steam account in a will

I wouldn't suppose that people are required to inform steam that they're dead. Therefore, I'd assume the easiest way to bequeath games/DLCs, etc, is to get a wishlist from your loved ones, and then gift all of those games prior to death on a credit card that you might not be able to pay, due to being dead. Steam gets the money, the CC company gets shafted. Alternately, share your credit card details with a loved one and that list, and have them order within hours of your death (this depends on whether or not you were plausibly alive when those CC transactions took place)