It was 2.3 I think, so around November 2021.
I got used to the change pretty quick, but I never really ran Nihlathak much either so the muscle memory wasn’t too strong. 
I was in support of its removal back then but looking back I wouldn’t mind if they had kept it and instead just made the visuals more accurate to what was happening so that new players wouldn’t get demolished by what they assumed to be poison (or nothing in the case that the poison cloud visually dissipated) and wonder what the hell happened. 
I don’t mind that it’s gone either though.
Yep, was indeed 2.3 released Dec 2021:
Enemies
- Fixed an issue where Tomb Vipers’ Poison Cloud attack was improperly dealing physical damage.; the poison clouds now deal proper poison damage