In 5e, most things are 1/2 damage or immune to damage (the feat Heavy Armor Master is -3 damage while using heavy armor).
Yep. Immunity to slashing/piercing/bludgeoning is extremely uncommon. Even a completely incorporeal creature like a Ghost only has resistance to normal weapons. For challenges up to level 10 I seriously doubt we'll run into anything that's immune to standard weapon types.
