What are the current restrictions on iOS emoji usage?
Whenever in such doubt when publishing an iOS app, your first step should be to thoroughly go through the App Store Review Guidelines.

Searching for emoji, these are the relevant matches in the document:


  4.5.6 Apps may use Unicode characters that render as Apple emojis in their app and app metadata. Apple emojis may not be used on other platforms or embedded directly in your app binary.





  5.2.5 Apple Products: ... Apps and extensions, including third party keyboards and Sticker packs, may not include Apple emoji. ...


It is clear that using unicode characters which render as Apple Emojis is permitted, but using Apple Emojis is not allowed.