I often heard about the statement that sandbox solutions are deprecated in SharePoint 2013, but I didn’t see any official reference from Microsoft about this. But here is one:

Apps for SharePoint compared with SharePoint solutions

In the “Important” box at the beginning, we see the statement from Microsoft:

SharePoint sandboxed solutions are deprecated in SharePoint 2013 in favor of developing apps for SharePoint, but sandboxed solutions can still be installed to site collections on SharePoint 2013. […]

But I am wondering, why we see a statement SharePoint Sandbox isn’t Dead…UserCode is in an MSDN blog.