I might quibble about gnolls being demonic in 4e, but that edition certainly played up their worship of Yeenoghu and attributed to them a savage, unrelenting brutality. 5e pushed that narrative even farther by making them literal demonic creations - basically uplifted hyenas.
And yeah, all of this is a pretty weird exception to WotC's treatment of other humanoids these days.
Its debatable as they are described in some places only as demon worshipping mortal instruments touched by him and not directly demons themselves. But there is 4e lore on Yeenoghu's influence on the entire race and there are a bunch of demonic gnoll monster entries.
The 4e MM "As the mortal instruments of the demon lord Yeenoghu, who is called the Beast of Butchery and Ruler of Ruin, gnolls constantly perform atrocities. When not scouring the land in Yeenoghu’s name, gnolls fight among themselves and participate in rituals that involve acts of depravity and self-mutilation."
From Dragon 364: "The Beast of Butchery was not always worshiped by the gnolls, for long ago these disparate tribes paid homage to a now almost forgotten god named Gorellik, their maker and the patron of hunting, beasts, and the wild. After murdering Gorellik during the ancient wars between the primordials and the gods, Yeenoghu absorbed the fallen god’s essence and laid claim to his children, forever after staining the race with his own foul corruption."
Dragon 367: "Gnoll legends trace the origin of their species to the demon prince Yeenoghu. Known as the Beast of Butchery and Ruler of Ruin, 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. Through this butchery, they earn a place of honor in the domain of the demon prince.
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, it is easy to see the inspiration for the myth. A gnoll looks much like a humanoid hyena and shares many traits with the animal, including its haunting laughter. And the strength and savagery of these creatures is enough to suggest demonic blood; the typical gnoll band strikes without fear or mercy, tearing into enemies with the fury of fiends incarnate."
