I think this is getting at the more robust solution.
When youtube videos are taken down or made private, they are not deleted necessarily from my playlists. There's simply an indicator that they video is 'no longer available'. I don't see why something similar can't be the behaviour for GitHub stars and watches?
I recently discovered a neat trick that if you Google the url for the private video (you still have this in your list), you're likely to find the title of the video, therefore allowing you to at least figure out what it was and seek out a new video.
When youtube videos are taken down or made private, they are not deleted necessarily from my playlists. There's simply an indicator that they video is 'no longer available'. I don't see why something similar can't be the behaviour for GitHub stars and watches?