People in low elo only complain about him being OP because he's popular. There are way more broken champions in the game: i.e. Talon, Camille, but are rarely played in Silver so you don't see anything on boards.
Also he's bad for reasons that have nothing to do with his ability to 1-shot someone. He overkills people with his ulti, but is pretty bad without it (they keep siphoning power into his ulti away from his kit).
So the idea is that: It's not hard for Zed to become very fed (10+ kills) - which makes silvers lose their mind as they die. But if you have any clue what you're doing, the 10+ kill Zed will still be worthless in determining the outcome of the game. Like you see Faker play Zed and be 20+ kills, but still seem to have little impact. Then take most other "snowbally" champions like Jax, Lucian, Camille, Akali, Ekko etc... and give them 20 kills, and the games absolutely over.
But in lower elos, people have no idea how to group and abuse, so Zed just getting fed leads to people running mindlessly in their jungle and dying 1v1.
So pretty much this: Zed's always been complained about by low elo, like every assassin in the game. He's complained about more only because he's popular. If Ekko was as popular as Zed you would see the same amount of complaint threads.
He's bad in high elo... because he's just bad. He doesn't have the tools to close the game with his lead, because he puts himself in too much danger. A 5/0 Zed and a 10/0 Zed just don't really accomplish anything different. They will 1-shot you with ult, and not accomplish much without. Which really sucks for a player trying to carry.