Assuming we have launched an app and filled the app with lots of data.
What happens if an object class is deleted?
What happens if a property of the object class is deleted?
Is there a way to reverse this?
Can I access the data that was defined by the object class/property that was deleted?
First of all, to delete an object class from the data model you need to remove all its references. So if the object class is in use somewhere in the solution, you cannot delete it until you remove all its references. The same applies to properties.
If there are no references to the object class (or property) and you delete it, you will not delete the data stored in the object class. So if you need to reverse that action, you can roll back to a previous snapshot of the solution (see more about snapshot here). Note that only the solution (everything you have created in Appfarm Create) will be rolled back - not the application data.