Zed shouldn't have good escape to begin with.
I mean if you delete his center of his entire theme and playstyle, that being his shadow, yeah, he "shouldn't have good escape." But then that's not Zed, so I'm not sure that's entirely realistic. We can't just make him into a second Talon. That would be silly.
He is an assassin his escape should be you dying.
That's not really an escape. Being under turret or among teammates wouldn't allow him to escape if he kills his target, right? Those two principles are just...entirely different things.
Burning his escape is part of the combo.
Yeah, and herein is the counterplay. He has a long cooldown tied to his shadow, which means if he uses it offensively, it's not up to use defensively, and vice versa. This makes his choices interesting for both the Zed player, and the person he is playing against. Now we're gettin' somewhere.
And again, zed is an assassin he should be easily harassed away by anything with half decent poke unless he can get in killing range at pop the target before they can turn it back on him
You keep using that word, "should."
Look, things aren't so black and white in reality. Characters can be members of a given class without having every single component of it in their kit. They can also have characteristics of other classes, and still be a different class primarily. Look at Vlad, who is described as a "tank mage" of sorts, or compare Corki, a "caster ADC," with Caitlyn, an ADC whose abilities serve to mainly supplement her auto attacks. Etc.
Like, Juggernauts are defined by having 1) low mobility, 2) little CC, 3) melee auto attacks, 4) some inherent sturdiness, and 5) lots of damage. Doesn't stop Garen from having a movement speed buff and hard CC in his Q, or Darius from having a displacement and a slow. Or Skarner actually being pretty quick in general and having his ultimate.