There has been a lot of speculation and confusion around sandbox solution being deprecated in SharePoint 2013. But finally, the SharePoint team has clarified that no-code sandboxed solutions (NCSS) are very much supported.
Following is the update from the SharePoint team. In conclusion, we have some clear message coming across from the production team w.r.t sandbox solution development.
Sandbox Solution Development – Conclusion
|Sandbox solution without managed code|
|Sandbox solution with managed code||Not Supported (Use the SharePoint App Model instead)|
Reference - SharePoint Dev Blog