So was reverting kog a nerf or a buff?

nerf. its not a full revert, there are mostly just removing his ability to break the AS Cap and removing the double AS on his w. There is some compensation to his damage but it is a nerf.

lets call it an adjustment. They will buff him to compensate, but we need to wait.

Well kog was basically unstoppable after 3ish items as long as there wasn't a fed assassin on the enemy team or something, so it's a nerf in that he's not necessarily OP now.

Whether he's still OP (when played well) or in need of buffs or actually balanced is yet to be seen. He's probably much more mechanically demanding now though (kiting is a thing again, and probably weaving abilities between attacks for shen procs if he builds that), so there will likely be a big disparity between the win rate of inexperienced kog players and the win rate of more experienced and mechanically gifted players, and so the impact on his overall win rate will probably not tell the whole story.

Not a nerf.