I've setup a github robot user named lssttsadmin and added a github oauth personal access token to jenkins for management of webhooks in the future. I'm actually not sure at this point if the errors being displayed were current or not... manually triggering a re-registration of webhooks resolved a few of the repos. Most of the rest were resolved by removing the webhook from each repo and allowing jenkins to recreated. However, there are still 4 repos remaining. Two are outside the lsst-ts org and were coming from a job named DockerBuildTest, which I have removed.
At this point, there are no longer any webhook registration errors appearing in the log after triggering a re-registration. I suspect this is merely failed state being kept by jenkins and the errors for repos not currently in use by a job are never going to go away on their own. I've got a recursive grep running trying to figure out where jenkins is storing this state – it isn't anywhere obvious and I'm wondering if it is actually greping the logs itself.