git push backup master
Where backup is a remote that exists literally anywhere else. Amazon, Gitlab, and Bitbucket are three choices that come to mind.

Sure, I know how to push stuff somewhere.

My question is more about the where. What is a good free service that is expected to stand the test of time?

Maybe it's possible to make archive.org archive GitHub repos?

Sure, you can use `git bundle` to export your repository to a file, then upload it to the archive. There's a nice command-line tool for doing the upload (https://github.com/jjjake/internetarchive), so you can easily script this so that it's automatic.