The bivy itself isn't mandatory, at least not here, but digging is...
Bivy helps to keep sleeping bag dry in a wet snow, other than that, you would be much happier with the same amount of weight as added feathers. That is unless your sleeping bag is not windproof which it should be. If the wind can get really high, like in the mountains or in outer achipelago etc. you might still want the bivy.
Digging reduces windchill and does not have to weight anything extra, so it's stupid not to dig. Tamping, well not mandatory. It just reduces the amount of digging.