Storing a json string that is 10 kb in size in a SQL Server CE cell
Just because something is marked as deprecated doesn't meant that it's disappearing soon - there are features that have been marked deprecated for several versions which haven't been removed yet. That said, you're right to move away from deprecated features if the opportunity presents itself (but don't make changes simply for the sake of change), and you should definitely avoid implementing anything new using a deprecated feature.

Per Microsoft's documentation on ntext, the replacement for ntext is nvarchar(max).

You may also be interested in reading about SQL Server 2016's support for JSON data.