So how come Akali can't dash through Azir's ult

Please. Malphite can't even ult through it. I'm pretty sure Sion is stopped as well.

Because Azir's wall is also coded to "STOP ALL DASHES", It stops J4 E-Q combo, it stops Akali dash, it stops Graves dash, it stops Lucian dash, it stops ALL DASHES.

It's not a wall, more of a barrier. Only thing that can get past it is blinks, unstoppable forces, and for some reason Fizz's pole jump.

It's part of his skill. It pushes back those that try to dash through it. You can still use blinks over it though. Like flash.

The reason people can't dash through it is because upon contact with it causes your champ to be knock back. While a dashing champion ignores terrain they are still targetable and hard cc will interrupt the dash (Think of how Janna can stop dashes with her tornado). So yes Azir's wall is a wall however it's a wall that ccs you when you touch it.

It stops all dashes except CC-immune ones. So can still ult through it, and can black shield you for it.

When thresh lands his hook and than wants to pull himself to azir through the wall. He also is getting cancelled.

No, it's not just a wall.

It's a knockback effect that is immediately in order if you move within the knockback hitbox (which is within the soldiers, and you cannot walk up to them because you are blocked by a "wall").

As Akali dashes, she touches the knockback hitbox, and due to animation order her ult is interrupted and she gets knocked back.

Fizz is not affected by knockback because he is invulnerable while in E and therefore unaffected by the knockback hitbox (not the same with Q).