Back in December, Mark Gurman of Bloomberg reported the existence of Marzipan, which is said to be an internal Apple project that would allow individual applications to be designed for the Mac and iOS user interfaces. In January, Gurman followed up to point out that the project was on track to be included in iOS 12 and macOS 10.14 this year. But according to veteran Apple observer John Gruber of Daring Fireball the initiative is unlikely to come up this year, and may be less ambitious than previously assumed.
From its sources, Gruber says that the project is not its codename is Marzipan and "sounds like a declarative control API", which is not necessarily related to multiplatform development, but theoretically it would allow to build applications for multiple interfaces of user at a time. However, this alone would not be something to help developers to port existing iPad applications to the Mac, for example, since applications would still have to be encrypted for each platform.
It makes sense that if Apple believes that (a) iOS and MacOS should have declarative control APIs, and (b) they should address the problem of abstracting API differences between UIKit (iOS) and AppKit ( MacOS), they would address them at the same time. Or maybe the logic is simply that if you are going to create a cross-platform user interface framework, the basis of that framework should be a declarative user interface.
Whatever marzipan ends up being named, Gruber says he is "almost sure" that he will not appear at WWDC next month, and he doubts it was a 2018 project at the time of Gurman's initial report . WWDC 2018 begins on June 4, so we will not have too much time to find out.