Ever since s3 when Renekton was the undesputed god of toplane, riot have been very careful with over buffing the guy. If you take a look at previous patch notes, you'll see that riot is very conservative with the buffs that they give him. As a result, renekton has actually been a medicore pick for the majority of his life cycle. Rarely is he a top tier pick, and even when he's good there's usually some other better toplaner that you can play.
It's safe to say that renekton is acutally one of the most fair / ballanced toplaners right now. He's good at what he does, which is dominating the early game, but his weaknesses allow supports and other peel champions to make quick work of him come mid-late game, when everyone else starts to get their items.
Riot gave renekton those buffs because of the rising popularity of ranged toplaners at the time. Gnar, Lulu, Ryze, Lissandra, Swain, and Kennen were all frequent picks in the toplane, and Renekton had the unfortunate problem of not only getting bullied out in lane, but also getting outscaled by the majority of other toplaners at the time.
When lissandra can simply press root you every time you try and dash at her, Swain can 1v1 you in meele range once he hits 6 while also poking you down from range, and Gnar just has to hop his happy little butt away every time you try to dash forward (attempting to double dash at him just puts you in his minion wave with no escape. Have fun getting kited back to your tower), something was wrong. The buffs to W were to hopefully let renekton land his stun a bit easier. His counterplay in these matchups was to let these champions push him in and hopefully land his stun on them so he could chase them down the lane for a kill.
As for renekton now. He mostly became strong because of indirect buffs and meta shifts. Botrk is amazing on him. BC is still great on him even after the nerf. Meele toplanners are becoming more frequent once again (Darius, Fiora, Rumble, ect.)
He's a good pick right now, but not the best. He still has to compete with the likes of Kled, Quinn, and Darius. He's always going to be an annoying lane bully in the early game, but I wouldn't waste a ban on him.