We have introduced the new SharePoint app model as a replacement to those scenarios that required the use of managed code. The app model decouples the SharePoint core product from the app runtime, and this enables much more flexibility and gives you the ability to run the code in the environment of your choice.
We realize that our customers have made investments in coded sandboxed solutions and we will phase them out responsibly. Existing coded sandboxed solutions will continue to work in on-premises SharePoint farms for the foreseeable future.
Given the dynamic nature of online services, we will determine support needs for coded sandboxed solutions in SharePoint Online based on customer demand. NCSSs continue to be supported. All future investments will go to making the new SharePoint app model richer and more powerful.
Accordingly, we recommend that all new development should use the new app model whenever possible. In scenarios where you have to develop a farm solution or coded sandboxed solution, we recommend that you design it so that it can easily evolve toward a more loosely coupled development model.
For more information about the app model written especially for experienced SharePoint developers, see Reimagine SharePoint Development.
Call to action: We want to make the app model great. Help us by identifying scenarios, APIs and feature gaps that make farm solutions or coded sandboxed solutions necessary today. Provide your suggestions and ideas in our UserVoice site.