We recently added support in Jenkins jobs for passing an auth token along with requests generated by Composer to the GitHub APIs (T106452: Composer activity from Cloud VPS hosts can be rate limited by GitHub). This seems to be working, but it is also likely to stop working on or before November 13, 2020. This will be caused by an upstream feature deprecation at GitHub.
The fix needed on our side is to upgrade Composer to a newer version which is following the announced change. Composer version 1.9.3 includes the required changes. It has been nearly 2 years since we updated Composer (T125343: Upgrade integration/composer to 1.6.5 stable), so it is probably past due anyway. At the moment I write this, v1.10.1 is the most current Composer release.