When iMessage applications were announced during the WWDC 2016 keynote, I didn’t expect too much. Stickers, message effects, and maybe access to the most common APIs. Generally, features that only big players will make good use of. Thankfully, I was wrong because the reality is completely different.
According to messages, there are no restrictions at all. Sure, your application can still be rejected during the App Store review. But, if you wish to, technically you can implement any regular iOS application as an iMessage application. Even things like in-app purchases. I think it’s amazing because it opens up a whole new market for developers.