Problem with thresh is that he actually is OP. Most people don't see it because he doesnt get 45 kills everygame. This guy has just too much in his kit with a decent damage. Think about it : Hook for 1.5 seconds (and yourr moved) then knock air bone for 0.5 seconds then slow for 1.5 seconds then 99% slow for 2 seconds.. Its 5.5 seconds CC while hes tanky af and his first hit on e can deal 200% of his ad (usually if you want an aggressive lane early game youll run with like 6-8 ad on thresh, this is what Lemonnation does). His early damage potential is hi because nobody is tanky except braum and late game he has idiot low cooldowns on his abilites. He also grants a shield with an escape that is way better than flash because it doesnt matter if you are rooted itll grab you anyway.
Finally hes been top 3 support since he was out but only 1 nerf jsut a little bit (annie massively nerfed, braum very bad when he was out, morgana huge nerfs on her, karma full rework that destroyed her support role and tahm needed major buffs too, also Bard kept getting buffs since he was out, alistar was always trash until they overly buffed him to nerf him after and nautilus before his mana rework he was bad too).
But thresh? no touch to his hi mana, neither his low cooldowns. His only nerfs he received were on his souls (lower rewards but its so bugged it doesnt matter) and his dark passage shield (2 people max included himself)