Turning off YouTube Short Remix on all Your Videos at Once
Go to YouTube Studio in your browser (no way to mass edit in the app)
Click on "Content" on the left pannel.
On the left just above your latest video, click the check box that has "Video" beside it
Click "Select all"
Click "Shorts remixing"
Click the pull down tab that appears
Click "Don't allow remixing"
Click "UPDATE VIDEOS"
If you have a lot of videos, this may take a few minutes.