Is the IntegrationApi 1.1 shown at https://appstore.sdl.com/language/developers/sdk.html the current one for Studio 2021? If not, where can I find documentation for the 2021 version?

I specifically need help with updating plugins to 2021 that have ViewParts that do not use WPF Controls.

Previously, it was possible in a class extended from an AbstractViewPartController, to return a System.Windows.Forms.Control as the return value of the GetContentControl() method.  Now, the return value has to be an IUIControl.  Is there a way to return a Control that is both a System.Windows.Forms.Control and a IUIControl and so that the AbstractViewPartController will have access to the elements of the control?  Or does the 2021 version of ViewParts only work with WPF Controls?