Yes.
The ability to kill every NPC we come across. Bring back Biff the Understudy and his clones in case it's a plot critical NPC.
And if you're worried that it might be unimmersive/breaking the 4th wall, I have two arguments against that:
1) it's not more unimmersive then untouchable/invincible NPC's
2) players that would kill any PNC (especially plot-critical ones) are well aware of what they are doing, and are willing to the pay the price of 4th wall breaking
They've said you can kill almost everyone including PNCs, but with a couple exceptions like Gods and Undead (I took that to mean super powerful undead, not like zombies, but someone like Larloch or Szazz Tam). That doesn't automatically mean you can't attack them, it could be they are so powerful they will crush you like bug for 100% certain, BG3 only goes up to level 10 after all.
