Replace web parts in sandbox solutions
One of the reasons many developers have leveraged code-based sandbox solutions is a desire to utilize visual web parts. This provides a great way to separate code from layout as well as utilize the ASP.NET controls. You can continue to use visual web parts in a provider-hosted add-in via client web parts. This is a great approach and provides a direct migration path for many applications.
Code-based sandbox solutions were deprecated in 2014, and SharePoint Online has started the process to completely remove this capability. Code-based sandbox solutions are also deprecated in SharePoint 2013 and in SharePoint 2016.
Options for replacing web parts
|Design considerations and more information
|Provider-hosted add-in client web part
Removing sandbox code from your site
When you deactivate your existing sandbox solution from your sites, any assets or files deployed by using declarative options are not removed. However, the features in the sandbox solution are automatically deactivated and the event receiver is removed.