so now I'm starting to wonder if Microsoft introduced some way to replace this "deprecated" SetDragRectangles?
It has been replaced by InputNonClientPointerSource
I tested by updating MSDN sample and it seems to work (still on Windows 10 only)
IntPtr hWndMain = WinRT.Interop.WindowNative.GetWindowHandle(this);
Microsoft.UI.WindowId myWndId = Microsoft.UI.Win32Interop.GetWindowIdFromWindow(hWndMain);
var incps = InputNonClientPointerSource.GetForWindowId(myWndId);
// In SetDragRegionForCustomTitleBar, same MSDN code with dragRectsList... dragRects
incps.SetRegionRects(NonClientRegionKind.Caption, dragRects);