I will simply quote this:
During a Q and A session at the Build 2014 Conference, Microsoft explained that Windows Forms was under maintenance mode, with no new features being added, but bugs found will still be fixed. Most recently, improved high-DPI support for various Windows Forms controls was introduced in updates to .NET Framework version 4.5
That means no more "new features" will be expected from Windows Forms. But yet Bug fixes and improvements can be released from time to time.