There is no guaranteed backward compatibility between classic and the new app. There are features that have been removed altogether. And each version of the app has a feature called “smart home monitor” but they are completely independent of each other and work somewhat differently. There are other incompatibilities as well.
See the following thread for that discussion (the title is clickable link).
As far as the new development platform, no guaranteed backward compatibility there, either. But it’s not finished yet, so all you can do now is test stuff with it.
And the last time I looked, one of the things that was unfinished was any way to communicate with A zigbee or Z wave device connected to the hub. That is, they have not yet released the details of whatever the new equivalent of device type handlers will be for those protocols.