Back in December, Bloomberg's Mark Gurman reported on the existence of Marzipan, said to be an internal project at Apple that would allow for single apps to be designed for both Mac and iOS user interfaces. In January, Gurman followed up to note that the project was on track for inclusion in this year's iOS 12 and macOS 10.14. But according to longtime Apple watcher John Gruber, of Daring Fireball, the initative is unlikely to surface this year — and may be less ambitious than previously assumed.
From his sources, Gruber says the project is no longer codenamed Marzipan and “sounds like a declarative control API,” which doesn't necessarily relate to cross-platform development but would theoretically allow apps to be built for multiple user interfaces at once. This alone, however, wouldn't really be something to help developers port existing iPad apps to the Mac, for example, since apps would still have to be coded 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 the API differences between UIKit (iOS) and AppKit (MacOS), they would tackle them at the same time. Or perhaps the logic is simply that if they’re going to create a cross-platform UI framework, the basis for that framework should be a declarative user interface.
Whatever Marzipan ends up being and being named, Gruber says he's “nearly certain” that it won't appear at WWDC next month, and doubts it was likely to have been a 2018 project at the time of Gurman's initial report. WWDC 2018 starts on June 4th, so we won't have too long to find out.