When you start recording with MovieRecorder or MovieRecorder Express in macOS 10.14 or later, you may see the following warning:
This is because, for QuickTime Classic destinations, we still use some QuickTime APIs that will not work in future versions of macOS.
This will be changed when these versions of macOS will be released, or even before. So you can safely ignore that warning for now. It will be shown every time you update MovieRecorder though, but it is not blocking and your recording will start anyway.
The warning won't show up for None, Segmented, MXF or Streaming destinations.