I've just discovered that one of our old users (barrycarter, now "user21"), who is no longer with us, used a personal, unlicensed github repository in 31 of his posts: https://data.stackexchange.com/astronomy/query/1292675
It seems he has since deleted his account with github, and his repository along with it. While his answers do not technically rely on the contents of the repo to understand the answer (in at least the handful of cases that I checked), they do rely on the repo for users to be able to validate his answers in many cases.
This presents an interesting problem. In a sense, this is a subset of the broader issue of link rot, but it is unique because he never licensed his repo, which means that he still holds full copyright on that repo; and even if someone had saved a copy it would be illegal to share here.
This practice feels antithetical to the mission of Stack Exchange in creating an enduring knowledge base. Should we create clearer guidelines for the use of links to user-created external resources in posts here? If so, what should they be?