So far, Apple have only announced Declarative Management for User Enrollment, not for Apple MDM in general
The SM team, however, are very interested in this, in that it allows changes to be made instantly on the device (within reason) and also allows the device to tell Systems Manager that something has changed WHEN it actually changes.
I personally feel that Apple are dipping their toes in the water to see what the take up and interest will be before deploying this for all types of management.
The other problem is that, Systems Manager, as well as other MDM platform, have complex business logic engines to device what should happen to devices when something is reported back or has changed. Because the devices now have the ability to have business logic applied on them, you're having to split the logic between the MDM platform and device. This isn't easy to do. And if you wish you let admins chose between SM doing the logic, the device, or a bit of both, it magnifies this complexity even further!
Having had a look at Apple's documentation, there appears to be limited support for this for a user enrolled device
If you could let us know what features you're interested in, that would be immensely helpful, though!