First and foremost, anything pertaining to Pokemon XY specifically should be posted in the XY forum. Pokemon General is for things that apply to the entire broad topic of Pokemon (ex: "What is your favorite Pokemon" or "Which is your favorite shiny"), or anything else regarding Pokemon that doesn't fit in any other forum we have. With this case though, we do have an XY forum which would definitely be the best place to post a question like this.
Also, you should post these types of questions in the Quick Q/A thread, as a question like this doesn't really need to have its own topic about it since it can be answered in just one reply.
Finally, for the question itself (before I lock this), I believe the answer is "Yes, Zygarde is shiny locked" after a quick google search regarding the subject.