Why Darius is still banned?

I will not deny that Darius is powerful, but he doesn't deserve "pre-silence removal Kassadin" levels of ban.

Maybe he deserved before, but the last nerfs on my opinion made him much more balanced. He now has to actually hit the blade to do significant damage and heal himself. So you have two options: either you try to stay away from the blade's range, or you stick to Darius so that he loses most of his dueling potential.

Not to mention, Darius tends to lose lane to most ranged champions. Lulu for example, will not only crush him in lane, but she will also ensure that Darius will never be able to do anything during teamfights. Gnar can do the same as Lulu with his CC and kiting potential. Renekton is a good option too because of his hit-and-run tatic, he can outsustain Darius while avoiding extended trades with him. There is also Yorick, he can definitively outsustain Darius while kiting him.

I think that the main problem is that people try to engage in long duration trades with Darius. However, peharps with the exception of Jax, Darius is THE master of extended trades. You can't outduel him, especially if you allow him to hit the blade and sustain himself. Darius will win trades with anyone during the early game, and will potentially still outduel most champions during the late game. To combat Darius you should kite him, poke him, and never, ever, try to directly 1x1 him. If possibly, focus on farming. Asking for your jungler's help is also a good idea because Darius lacks escapes. Think of Darius as pre-nerfs Jax, except that he is immobile, but has sustain. Against pre-nerfs Jax you would just avoid extended trades because Jax would always win them.

At the worlds, Darius was almost always banned, but almost everytime he was open and picked, the enemy team would pick Gnar, which can counter Darius since he can kite and poke him. Darius was also constantly ganked and he would need his team's help to do something relevant during teamfights.

On my opinion, if Riot needs to nerf him, I would suggest give to Darius higher mana costs. This will nerf his early game potential while making him unable to spam his axe that much, limiting his sustain. Look at Galio, which has enormous burst, harass and sustain potential, but is severely gated by his mana costs.