He was nerfed bc, people are dumb that simple.
Donger by champion design will never get a single kill if they enemy doesn't make mistakes. All he can do is push a lane. Players are dumb and die going in on him, its much simpler to just kill his turrets. While the new runes give him plenty of mana they still have a cd and worth gold. Sure lv1 hes oppressive but level 3 hes simply outclassed people are just to gun ho attack going in. Learn to play back and go in after you delt with the majority of his defences.
Also you can kill the R turret. People just tend to run away allowing it to attack them an exstra time and kill them.
Play it slow and donger at max will have 1 reg turret, 1 ult turret. rockets and stun for a defence instead of 3 turrets. Which means 1 charged beam if he lands the stun. If his rockets go on cd from a bait, then you've taken out half his damage and only need to worry about killing the ult. Don't forget Donger is burst only not sustained dps. kill turrets blow cds and he'll cry all the way to gg. And that was before the nerf.