Observed in JX's updatebot rather than Jenkins, but might affect Jenkins uses as well (GitHub multibranch etc. etc.):
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.kohsuke.github.Requester (file:/usr/bin/updatebot) to field java.net.HttpURLConnection.method
Indeed it seems that some code in this library digs into JDK implementation details, evidently to work around JDK-8207840. Maybe github-api should just use a better HTTP client.