No, none of the sliver commanders or non-legendary slivers are banned in Commander
It got nerfed during the July ban list changes by reducing it's toughness from 3 to 1, making it easier to remove not only by damage but also from cheap black removal like Cut Down
When I think about a human lifetime I often think in terms such as Worth, Value, Cost, and Price
While it can be argued that a card like Eternal Witness will often work better in some decks than Snapcaster Mage, you cant use that card to give something like Mana Drain flashback at instant speed to stop a key threat and keep you in the game
In fact, thinking about it more Im fairly confident that more different decklists will contain at least one Deicide in the 75 than any other card in the set, so I suppose its clear that Im not giving full credit for sideboard inclusion
If the player drew lots of cards in a Commander game, Brightstone Ritual is the best way to recklessly cast them all in one turn