Ah okay: I never played 4E but I knew that they did something with the Gnolls that wasn't exactly like 5E's version. I know 4E Gnolls, those that willingly choose not to serve Yeenoghu, do have to worry about giving in to the Demonic Corruption of Yeenoghu. So they are capable of rejecting it, it's just that at times it can be super annoying/hard at times.
I still don't get WoTC changed Gnolls in 5E to basically be unrepentant killers.
Pre-4e the gnoll's origin was unknown even to the gnolls themselves. None of their previous pantheon actually claimed to be their creator deity. That helped Yeenoghu to lure the gnolls away from their gods, who then withered and died save for a few leftovers desperately hanging to the last crumbs of divintiy (chief among them Gorellik, but 2 others were later mentioned IIRC). The gist was that Yeenoghu ensnared the gnolls into his worship, but he had no true claim to them.
In 4e they were apparently unhappy with that background and made them into even more mindless beasts of Yeenoghu.
First words from 4e Monster Manual about Gnolls:
Gnolls are feral demon-worshipping marauders that kill, pillage, and destroy. They attack communities along the borderlands without warning and slaughter without mercy, all in the name of the demon lord Yeenoghu.
It does not get any better after that intro.
When they then released the Monster Vault as one of the core books for D&D Essentials (basically a D&D 4.5 to save the edition) it also didn't get better. It repeated what was already said in the MM and added stuff like
They follow the demon lord's edicts and the orders of his demons without question. When not marauding in Yeenoghu's name, gnolls fight among themselves and participate in ceremonies that involve acts of depravity and self-mutilation. Some gnolls are known to mate with demons or perform rituals that bind their bodies or souls to demonic forces.
I couldn't help myself but dig for when the part of them being an actual creation of Yeenoghu was first mentioned, as this is not in the orginal 4e MM and also not in the MV (although I knew it had to be from much earlier in 4e's lifetime than the MV).
So I just googled "Gnolls 4e" and looked at the sources listed and then found it.
Dragon Magazin 367, the article
Playing Gnolls
Yeenoghu sought to spread his hand across the mortal world by creating an army that would sow discord and terror across civilized lands. His demons numbered too few and could not remain in the mortal world indefinitely. But Yeenoghu took a few of his mightiest and most savage demons and fed them to a pack of mortal hyenas. The essence of the demons fused with the animals, and their children were the first of the gnolls—fierce humanoid creatures combining the traits of cunning hyena and vicious fiend. Yeenoghu charged the gnolls to spread horror in his name, bathing the lands in blood and bringing suffering to all things unfortunate enough to cross paths with the Children of Yeenoghu
This, to the best of my knowledge, is the first time that credit for the gnoll's origin has been attributed to Yeenoghu.
However, there's something more which I did not remember until this very research:
This is the tale gnoll mothers tell their pups and the story the scourge uses to exhort his troops. Whether it is truth or mere legend,
So while 4e brought this idea into official D&D lore, it did not fully confirm it. That honor does then indeed belong to 5e.