Hi Hebiki,
i will respond to all your questions. Ekko's last nerfs were on his base stats, exclusively to nerf the tank Ekko build. Riot said that when they designed Ekko, they wanted him to be an assassin. But these nerfs were making ekko worst either tank or assassin, so they increased the ap ratios on his spells, making assassin ekko, their goal, more rewarding than building him tank and buildhim tank close to useless. Also, now tank ekko falls off quite quickly. They also nerfed his W(the stun duration and the W passive damage). The nerfs made his early game way weaker, so its harder to get kills in laning phase and now you have to play ekko mid cautiously. So, no, hes not as relevant as ever, he was way stronger a few months ago.
Basically, his kit is strong because he is designed to be a squishy assassin. Assassins are by definition mobile and quick killers. Therefore, building him tank was broken because he has decent cc and good mobility, making him able to tank and dodge a lot of things and live forever in fights. The ability to dodge effectively some spells is for assassins, because they are squishy and need to make their way to their target without dying.
Now what is the relation between ekko's w having a passive and ekko's theme as a champion? What make you say that his w having a passive doesnt fit his theme? The reason why he has that, is because hes an ap melee assassin. Ap champions barely do any damage with their basic attacks but have strong bursts with their spells. You might have observed ( though im not sure if you actually do) that ekkos spells, besides his ult, do peanut damage. When ekko deals damage, its when he procs his passive, basically when he is hugging you and pretty fking vulnerable. So Ekko's W gives him strong basic attacks to reward him when he fights melee range, when hes dueling. As you can see, he doesnt have a passive on his w for no reason.
an 80% ap ratio on a passive (what?) AND a 60% slow AND an 80% movespeed boost.
As mentionned above, his passive is his main source of damage. Also mentionned above, this slow and this ms boost because he is designed to be played as an ap assassin. Assassins are meant to be sneaky
I'm fairly sure that's the biggest damage passive, slow passive, and movement speed passive in the entire game.
See above
Not to mention he has more burst damage then FUCKING ANNIE
Ekko burst damage (not even counting W passive): 1040 + 350% AP
Annie burst damage (one tibbers dot and basic, not counting E) 950 + 255%
Ekko has more damage than annie, yet still has a revive ult, an escape/gapcloser, two slows, and a movement speed boost.
FKING ANNIE is a mage, not an assassin