Enable Sandbox solutions on SharePoint 2016
Sandbox solutions and web parts are deprecated for a long time but due all the restrictions of the SharePoint app model caused the iframes this is still the best way to deliver rich web parts that fully integrate with the site and other solutions like custom themes.
Microsoft is pushing user’s way from this model by creating limitations to Sandbox solutions like removing sandbox solutions with managed code from Office 365 and deactivating the execution service by default on SharePoint 2016.