Developer of #1 iMessage sticker app - Phoneys - was served a notice by Apple regarding the nature of the app and given a week's period to change it.
Phoneys lets you place stickers that look like iMessage chat bubbles with different text in them in the conversation such that it looks to the other person as if they sent that text to you. It's sort of a prank app, which Apple no longer seems comfortable with.
After the app shot to the #1 position, the developer got a call from Apple's lawyer, where he was informed that the app was breaking some guidelines by using blue chat bubbles and Apple's San Francisco font, basically emulating the UI. Apple was also not fond of the "prank" nature of the app, although the developer never marketed the app as such. The developer is now given a week to alter the app to fit the guidelines, following which the app will likely be removed if no action is taken by the developer.
For his part, the developer was fairly understanding of where Apple was coming from, although Apple is partly at fault here as the app did go through its review process. Perhaps Apple underestimated how popular it would become and is now doing damage control. Either way, if you use this app, it will likely be changed or removed soon.