Backstab was changed to sneak attack in 3.0. Rogues are still the only class with sneak attack in BG3. Backstab in BG3 has nothing to do with 2nd ed backstabbing. It's not an equivalent kind of ability. There's no reason to say no class but rogues should get Backstab advantage just because Thieves had a completely different ability I 2nd ed that was called backstab. That's just... it doesn't even enter into the computation. It's irrelevant.
