On July 29th Microsoft announced that it starts with the removal of coded sandbox solutions. You can read the official announcement here. This type of sandbox solutions was already deprecated since summer 2014.
A coded sandbox solution contains a DLL. Sandbox solutions containing declarative XML (f.e. the definition of site columns and content types), CSS, JS, images, etc. continue to be supported.
It is recommended to start migrating coded sandbox solutions as of today! You can find a lot of alternatives and sample code based on recommended coded practices on PnP. Additional migration guidance has been published here.
If you’re not sure if your O365 tenant is impacted by this decision, you can run a script that generates a list of sandbox solutions that are installed on your tenant. The report also displays a flag that indicates whether your sandbox solutions contains an assembly or not.
It is possible that your sandbox solutions contains nothing but CSS, JS, images, etc; but that your sandbox solution is listed as containing an assemby. It is possible that your sandbox solution contains an empty assemby. In that case you can get rid of that assembly by simply setting the Include Assembly in Package property to false. This is a property on the Visual studio project itself:
This is also documented in this KB.