User blog comment:FairyRaven/Fairy Tail Chapter 329: Seven Dragons, Review and Commentary/@comment-7505640-20130501071155/@comment-2214433-20130501085909

We know that Yukino has mentioned "Zentopia incident" when she offered her keys to Lucy in manga.

Also, we know that the designs of post time skip appearances of Oracion Seis, Michelle and Mira's Halphas have been drawn by Mashima. So to a certain extent, it can be considered canon. At this point I'd call Key of the Starry Sky arc "anime only".