Over the past year, I've noticed that quite a few repos that I used to track have disappeared. I keep a local bookmarks list now because if a "starred" project is removed or DMCA'd, Github does not tell you about it and they remove any mention of the repo from the "starred" list.
Google's cached version of pages can be another useful option, if you click on the ellipses to the right of a search result's address and then "cached" in the bottom right hand corner of the "About this result" box.
Came here to post a similar sentiment. I've rescued very useful content from mirroring sites that was gone from the original. You can filter them out if you want, but don't forget you're doing that or you may not find what you're after.
