I have heard it was removed because adding AD and Armor and crit was an unhealthy combination on certain champs cough cough adc's cough cough
So why did the add ad to GA, remove the mr and make the health on revive scale with only base health?
Cause it's one of those rarely used items. So they got rid of it.
anti-melee bias
Atmas was going to cause issues at some point, especially with the sion rework coming really quickly
ADCs didnt buy Atmas Impaler...lol
They removed atmas because it was a useless stat combo for anyone except Garen. It was virtually never purchased, and when it was, 95% of the time it was either Garen or someone who misunderstood the item.
Atmas was never good on ADCs, and that wasn't the reason they removed it. It was only ever supposed to be a bruiser item and was only ever used by them, even when it was strong. It was replaced by titanic hydra, which is much more readable, easier to understand, and better suited to the majority of bruisers and health tanks.
It had nothing to do with ADCs, whereas GA was always intended to be an item for squishies getting bursted.
It enabled the old meta-golem builds back in the day (Force of Nature was a bigger part of that problem; but Atma's was also vital to the build), then got nerf batted pretty hard (Force of Nature was straight up removed and rightly so). Then was basically only used on a few champs because most of them either didn't want the crit, or didn't have the HP and desire for AR that would've made it worthwhile to consider.
Atma's was removed due to an increase in strength of champs that scaled with health at the time (
were some notorious examples).
Another thing was their dumbing down assassins that they were doing at the time ( Atma's was removed after worlds in season 4. In season 3 & 4 assassins were overpowered to the point where they could literally 1 v 5 even when behind. at this same time 
was getting nerfed, 
received a death nerf that made him irrelevant for almost 2 years, 
& 
all also recieved minor nerfs around this time.) they didn't want assassins building armor & getting free damage from it.
One last thing was their removal of snowbally items at that tim. there was a mass purge of items back then... (RIP DFG, Philosophers Stone, Sword of the Divine, among others).
At the time I was upset at the removal of Atma's as I was a heavy main at the time, but looking back it would have been an absolutly busted item in season 5's Cinderhulk tank meta...
