In the past there has been a lot of confusion around sandboxed solutions within SharePoint 2013 and SharePoint Online. A statement in MSDN uses the deprecated word that throws the usage of them into question. Thankfully last night we got some confirmation from Microsoft that clears it up.
The part that is deprecated is the user code part of sandboxed solutions. This is the managed code that you can include in your sandboxed solutions that runs in the sandboxed user code service. However, sandboxed solutions that contain only declarative things is fully supported and not deprecated.
Now THAT is a huge clarification. This means you can have supported declarative solutions deployed to the host web in SharePoint 2013. Customizations to the host web using the SharePoint App Model simply aren't possible in most scenarios so this is a welcomed clarification. It's too bad this took over a year for Microsoft to clarify this.
Looking for something to point to? Here's an official Microsoft source. I also confirmed this with a second source at Microsoft.
Now... if they'd only get their Office 365 updating process cleaned up and don't break working deployments out of the blue, we could get closer to a reliable platform in the cloud.