Pretty sure it was intentional. Read the patch notes.
Also blame Bjergsen. He said at All-Stars to the media that Zoe's damage is too reliable.
In my humble opinion, if they just nerfed her wave clear/push, Riot would not have needed to make her damage less reliable. The problem is right now is that Zoe out pushes all mid laners except for a few who are like wave clear monsters (xerath being one). She just nukes your wave and gets a free shot at you.
Healthy options in MY OPINION:
- Make her Q cost like 30% more mana so she has to choose whether to poke or push with it, can't do both constantly.
- Straight up nerf her wave clear by making her Q do less damage to lane creeps only. Like right now after just a couple levels she can 1 shot front line and 1 shot back line; make it take 2 Q's to burst front line (unless of course she is already fed).
You could have done either of these in conjunction with making it so Spellthief can't get Hextech Gunblade from creeps and she would be in a good spot IMO. Maybe keep the thing that shortens the delay from her exit portal on ult too.