Github has an issue where they no longer render (valid) notebooks: https://github.com/orgs/community/discussions/155944.
However there's a solution. You can simply replace github.com by colab.research.google.com/github in the URL of a notebook, and it will be properly rendered (and can be run) in Google Colab.
Hoping for someone at Github to fix this... if you have any other suggestions (other than removing all outputs of a notebook) let me know!