Answer by VonC for Finding the oldest commit in a GitHub repository via the API

One suggestion would be to list commits on a repo (See GitHub api V3 section), using the until parameter, set to the creation of the repo (plus one day, for instance).

GET /repos/:owner/:repo/commits

That way, you would list all commits created at the time of the repo being created, or before: that would limit the list, excluding all the commits created after the repo creation.