Since we probably won’t officially know until it happens, let’s speculate
Currently in the new app, you can create a scene and said scene can be activated by the Custom Automation smart app. Both of these are unique to the enw app and don’t show up in the classic app or the IDE.
But at the same time, custom smart apps installed in the legacy IDE and classic app, DO show up in the new app. So there is your continuing support of the legacy IDE. There are also marketplace apps from the classic app that have been migrated over (both SmartThings written and outside dev written)
But some solutions/apps aren’t in the new app (routines, smart locks, etc.). There’s another (Smart Home Monitor) that IS in the new app, but is a total re-write using the new API and has some new features. I have a feeling we will see more of this.