An organization I work with has a project on GitHub that no one outside the organization has used and we are end-of-lifeing it.
I see there is a great big delete button, but I wonder if there is a way to freeze the project so it can never be pushed to again, and write "decommissioned" on it in some way that will be universally recognized by other programmers.
First off, simply add an explanatory note at the beginning of the README.md file in the repo (and if that file doesn't exist yet, add it). Say something like "This project has reached its end of life, and is now just preserved here for posterity."
The non-enterprise flavor of GitHub doesn't let you disable push, unfortunately.